Airplane  Motors 

A Course  of  Practical  Instruction  in  Their 
Care  and  Overhauling 
for  the  use  of 

MILITARY  AVIATORS 
by 

George  E.  A.  Hallett,  A.  M.  E.;  A.  S„  S.  C. 


WASHINGTON 
Press  of  Gibson  Bros.,  Inc. 

1917 


Digitized  by  the  Internet  Archive 
in  2017  with  funding  from 

University  of  Illinois  Urbana-Champaign  Alternates 


https://archive.org/details/airplanemotorscoOOhall 

I 


rvs-5\ 


629-l^ss- 

\4  \S 

A PRACTICAL  COURSE  OF  INSTRUCTION  IN 
THE  PRINCIPLES,  OVERHAULING  AND 
CARE  OF  AIRPLANE  MOTORS. 

By  Geo.  E.  A.  Hallett,  A.  M.  E.,  Signal  Corps, 
Aviation  School,  San  Diego,  Calif. 


INTRODUCTION. 


Object  of  this  Course. — In  the  case  of  student  flyers,  is  to  give 
them  a practical  knowledge  of  airplane  motors  sufficient  to 
enable  them  to  diagnose  motor  trouble  when  on  cross-country 
flights  and  to  make  rapid  and  practical  repairs  if  possible,  or 
in  any  event,  to  be  able  to  send  an  intelligent  message  for  parts, 
etc.,  and  to  explain  to  mechanicians  nature  of  trouble  and  its 
remedy. 

In  the  case  of  the  mechanician,  the  object  will  be  to  make 
an  airplane  motor  man  out  of  an  automobile  mechanic,  or 
machinist.  Parts  of  it  should  be  useful  to  give  to  automobile 
factories  to  help  them  train  aviation  mechanicians.  The 
course  consists  of:  (1)  A series  of  lectures  beginning  with 
principles  and  covering  a wide  range  of  practical  work  as  well 
as  bench  and  block  work.  (2)  A bench  course,  of  overhauling 
unserviceable  motors  (preferably  airplane  motors;  methods 
explained  later) . (3)  Block  course  of  installing  motor  on  a test 

block  or  in  a fuselage;  placing  propeller,  cranking,  handling 
switch  and  throttle  to  start  motor,  carburetor  adjustment, 
“trouble-shooting,”  emergency  repairs,  inspection  or  “preven- 
tion of  trouble.”  (4)  If  time  permits,  a study  course  in  some 
good  gas  engine  book.  Attached  will  be  found  a list  of  twenty 
assignments  in  “Dyke’s  Automobile  and  Gasoline  Engine 
Encyclopedia.”  These  assignments  cover  the  automobile 
chassis  in  addition  to  motors  and  will  be  found  fairly  satis- 
factory. While  this  book  has  many  typographical  errors,  it 
is  fairly  practical. 


Methods  of  Conducting  Practical  Motor  Course. 

The  course  should  last  from  four  to  six  weeks  (18  hours  per 
week),  according  to  the  amount  of  material  or  motors  avail- 
/-able. 

(3) 


. >o 


4 

Three  men  on  a motor  is  an  ideal  number.  If  more  are  put 
on  one  motor,  the  work  progresses  too  fast  and  therefore  does 
not  “soak  in.”  It  will  be  possible  to  give  a ten  day  course,  all 
day  sessions,  which  will  cover  all  the  work.  (Note.— Each 
three  men  are  called  a “section.”) 

Always  encourage  the  asking  of  questions. 

The  First  Few  Days. 

The  first  lecture  should  be  given  on  the  first  day.  Other 
lectures  should  be  given  as  the  work  progresses.  The  first 
thing  to  do  is  to  spread  out  all  the  tools  on  the  benches  and 
name  them,  so  that  all  will  know  them  by  the  same  names. 
Give  each  section  a simple  motor.  (Note. — This  motor 
can  be  quite  obsolete,  and  not  necessarily  complete.) 

Explain  principles  of  disassembling  motor  as  the  work 
progresses. 

1.  Find  the  number  on  the  parts. 

2.  Decide  in  which  order  parts  should  be  removed.  (Note. — 
Some  parts  are  not  accessible  until  others  are  removed.) 

3.  Show  best  methods  of  removing  cotter  pins. 

4.  Make  marks  on  timing  gears  before  disassembling. 

5.  Keep  shims  and  liners  in  proper  place  (explain  impor- 
tance) . 

6.  Cleaning  parts  (especially  oil  passages). 

7.  Nomenclature  of  parts. 

Assembling. 

Explain  as  follows: 

All  parts  must  be  carefully  oiled,  because  the  oil  pump  and 
oil  pipes  are  now  empty,  and  it  will  be  best  to  keep  the  idea  in 
mind  that  an  engine  may  run  as  much  as  ten  minutes  before 
the  pump  can  deliver  oil  to  all  the  parts. 

Bearing  nuts  must  be  tight.  Never  loosen  the  nuts  to 
loosen  the  bearing,  but  add  shims  or  scrape  it  out  if  it  is  too 
tight. 

Explain  the  following: 

Making  paper  gaskets. 

The  use  of  shellac  and  graphite  with  these  gaskets. 

Grinding  valves.  (Note. — Grind  by  hand  and  lift  off  the 
seat  between  each  stroke.) 

Test  the  valves  with  gasoline. 

How  to  distinguish  the  exhaust  valves  from  inlet  valves.  • 
(Note. — The  exhaust  valve  stems  will  be  darkened  by  heat; 
sometimes  the  heads  are  designed  differently.  Exhaust  valve 


springs  are  always  the  strongest  if  there  is  any  difference  in 
the  two  springs.) 

Valves  should  never  be  ground  more  than  necessary.  If 
there  is  a fair  width  of  seat  all  around,  it  is  better  to  leave  a 
few  “pits’ ’ near  the  edges  rather  than  to  grind  so  much  of 
the  valve  seating  away. 

Explain  valve  timing  in  its  simplest  form,  viz.,  put  the  gears 
together  by  their  marks.  (Note. — It  is  not  necessary  to  put 
an  ignition  system  on  this  engine.) 

Have  the  class  trace  the  water  circulation  and  oil  circulation. 

Explain  how  to  determine  the  direction  of  rotation  of  the 
motor  by  watching  the  valves,  as  follows : 

We  know  from  the  lecture  on  the  first  day,  what 
must  occur  inside  a motor  to  make  it  run.  Remember 
that  after  the  exhaust  stroke  (recognized  by  the  exhaust 
valve  being  open)  we  have  a suction  stroke  (which  we 
recognize  by  the  opening  of  the  inlet  valve).  Therefore, 
when  we  turn  the  motor  in  the  proper  direction,  the  inlet 
valve  will  open  immediately  after  the  exhaust  valve  closes. 

The  above  work  will  consume  a different  length  of  time 
with  different  classes  and  motors,  and  therefore,  I set  no 
definite  time  for  its  completion. 

When  this  work  is  done,  give  the  class  motors  which  are 
complete  and  more  modern,  preferably  airplane  motors. 

It  is  an  immense  advantage  if  the  motor  can  be  installed 
on  a testing  block  or  in  a fuselage  of  an  airplane  and  run  on 
completion  of  overhauling,  because  the  class  will  show  far 
more  interest  and  effort  under  these  circumstances. 

As  the  sections  begin  work  on  these  motors,  make  them 
observe  all  the  precautions  taught  on  the  first  motors,  and  also 
teach  them  how  to  measure  and  find  the  valve  timing  (closing- 
position  of  exhaust  valve)  either  in  degrees  or  piston  position. 
In  airplane  motors,  the  valves  are  usually  timed  by  piston 
position. 

Explain  that  this  is  a motor  which  is  new  to  us  and  we  must 
know  the  valve  timing  in  order  to  be  able  to  put  it  together 
properly.  While  it  is  true  that  ordinarily  we  could  time  the 
valves  by  the  marks  on  the  timing  gears,  remember  that  fre- 
quently parts  either  of  the  gears,  the  crank  shaft,  or  cam 
shaft  must  be  replaced,  and  as  key  ways  and  gear  teeth  are  not 
cut  so  that  these  parts  are  interchangeable,  our  marks  will  be 
of  no  value.  Therefore,  we  must  know  other  methods  of 
timing  the  motor.  We  will  time  this  motor  by  degrees, 
preferably  by  piston  position,  as  we  assemble  it,  and  must 
know  the  valve  timing. 


6 

Explain  measuring  spark  timing.  (Find  at  what  piston 
position  the  spark  occurs.) 

Explain  how  to  try  the  bearings  for  looseness  before  the 
motor  is  disassembled,  so  that  we  will  know  how  many  shims 
to  remove  before  assembling  them.  This  saves  time,  because 
if  it  is  not  done  it  will  be  necessary  to  assemble  the  bearings 
before  deciding  how  many  shims  to  remove. 

Explain  that  they  must  note  the  gear  clearance  between 
the  crank  shaft  and  the  cam  shaft,  so  they  will  know  if  the 
bearings  must  be  raised,  or  possibly  renewed,  before  scraping 
them. 

Try  the  adjustment  of  the  thrust  bearings  and  oil  pump 
gears  before  disassembling  for  the  same  reasons.  The  thrust 
bearing  of  an  airplane  motor  must  hold  the  crank  shaft  so 
that  it  “floats”  between  the  main  bearings.  The  cheeks  of 
the  shaft  must  not  be  able  to  touch  the  cheeks  of  the  bearings. 

In  adjusting  a thrust  bearing,  if  there  is,  for  example,  a 
3 /64th  of  an  inch  end-play  in  the  crank  shaft,  the  thrust  bear- 
ing should  be  so  adjusted  that  it  will  hold  the  shaft  so  that 
2 /64  of  that  clearance  will  be  on  the  side  which  will  be  reduced 
by  wear  in  the  thrust  bearing.  Then  if  the  motor  is  a tractor, 
the  greatest  amount  of  clearance  should  be  left  on  the  side  of 
the  crank  throw  which  is  toward  the  thrust  bearing,  because 
the  pull  of  the  propeller  will  wear  the  thrust  bearing  in  that 
direction. 

Explain  the  testing  of  the  crank  shaft,  or  if  possible,  test 
the  crank  shaft  by  placing  it  in  a lathe,  between  centers,  and 
by  using  a gauge  to  determine  to  what  extent  the  crank  shaft 
is  crooked.  Explain  that  crank  shafts  are  not  used  if  they 
are  more  than  5 /1000  of  an  inch  out  of  line  or  crooked,  but 
must  be  straightened  or  reground. 

Then  carefully  measure  the  journals  with  a micrometer. 
The  journal  must  not  be  over  5 / 10000  of  an  inch  out  of  round. 
Crank  shafts  become  crystallized  in  time,  even  in  com- 
paratively heavy  engines.  For  instance,  the  big  omnibus 
companies  of  London  set  a specified  number  of  miles  that  a 
crank  shaft  of  any  certain  make  is  allowed  to  be  used  because 
it  is  found  through  long  practice,  that  if  used  longer  than  this, 
the  crank  shafts  are  liable  to  become  crystallized  and  wreck 
an  entire  motor  when  they  break.  At  present,  in  this  country, 
limits  are  being  put  on  the  use  of  certain  makes  of  crank  shafts 
in  airplane  motors.  , For  example,  one  crank  shaft  used  in 
the  service  must  not  be  used  more  than  three  hundred  hours 
of  flying  time. 


7 

Explain  the  alignment  of  the  main  bearings.  Emphasize 
the  fact  that  the  crank  shaft  is  limber,  and  if  we  should  bear 
down  on  it  while  marking  the  bearings,  it  might  be  possible 
to  mark  a bearing  which  was  already  2 /1000  of  an  inch  low,  etc. 

The  scraping  of  bearings  should  be  taught  because  it  is 
necessary  to  scrape  bearings  in  overhauling  a motor,  even 
though  they  are  usually  reamed  when  the  motor  is  built. 
If  possible,  give  the  students  some  old  bearing  caps  to  practice 
scraping  on.  Instruct  them  in  the  use  of  the  scraper;  warn 
them  to  avoid  starting  or  stopping  a stroke  abruptly  with  the 
scraper  because  this  leaves  little  notches  which  the  scraper  will 
jump  over  on  the  next  cut.  After  cutting  a series  of  parallel 
strokes  to  cut  away  part  of  a bearing,  the  scraper  should  be 
moved  across  the  surface  diagonally  to  smooth  it  up.  Point 
out  that  it  is  necessary  to  cut  comparatively  lightly  on  the 
sides  of  the  bearings.  For  example,  if  you  are  letting  the 
shaft  down  into  a bearing  1 /1000  of  an  inch,  it  will  be  neces- 
sary to  cut  a thousandth  deep  at  the  bottom  of  the  bearing, 
but  as  the  shaft  is  not  going  to  be  moved  horizontally  we  wull 
not  cut  any  metal  away  at  the  top  of  the  sides,  and  only  one 
half  a thousandth,  half  way  up  the  sides.  In  other  words,  cut 
deepest  in  the  bottom  of  the  bearing  and  lighter  and  lighter  as 
we  work  up  the  sides.  The  oil  circulation  of  the  motor  should 
be  systematically  traced  and  all  oil  passages  and  screens  care- 
fully cleaned. 

Cutting  Oil  Grooves. 

If  the  bearings  are  renewed  in  a motor,  the  oil  grooves  in  the 
new  bearings  should  be  cut  in  exactly  the  same  manner  that 
they  were  cut  in  the  old  ones.  The  form  and  location  of  oil 
grooves  are  usually  experimented  with  in  the  motor  factories 
and  should  not  be  altered  by  the  mechanic  who  does  the  over- 
hauling. 

Adjusting  Bearings. 

Before  deciding  how  tightly  our  bearings  should  be  adjusted, 
we  must  consider  these  things.  Have  we  a low  pressure 
lubricating  system,  high  pressure  lubricating  system,  worn 
bearings  (bearings  worn  smooth),  or  newly  scraped  bearings? 
The  modern  idea  of  bearings  in  an  airplane  motor  is  to  fit 
them  very  loosely,  from  two  to  four  thousandths  of  an  inch, 
and  maintaining  the  oil  film  by  forcing  a large  amount  of 
oil  through  the  bearings  under  very  high  pressure.  If  we  have 
a high  pressure  lubrication  system,  the  bearings  can  be,  and 
should  be,  fitted  loosely.  For  example,  they  should  be  care- 


8 

fully  scraped  and  fitted  to  the  shaft  and  then  a two  thousandth 
shim  placed  in  each  side  of  the  bearing  to  loosen  it  up.  If 
we  have  a comparatively  low  pressure  lubrication  system, 
the  bearings  must  be  adjusted  closer  or  they  would  hammer 
out.  If  we  have  newly  scraped  bearings,  the  shaft  will  touch 
the  bearings  only  in  comparatively  small  spots.  These  spots 
wear  rather  fast,  so  the  bearing  loosens  up  quickly  for  a short 
time,  until  the  shaft  has  worn  the  bearing  to  a perfect  “fit.” 
Knowing  this,  we  adjust  newly  scraped  bearings  tighter  than 
we  would  if  we  were  simply  adjusting  bearings  which  were 
already  worn  smooth. 

Instruction  in  filing  should  be  given  at  this  stage  as  it  is 
frequently  necessary  to  adjust  bearings  by  filing  the  bearing 
cap.  Impress  the  fact  that  nuts  on  all  bearings  must  be  very 
tight.  Point  out  that  on  a “V”  type  motor,  and  also  on 
the  vertical  type,  if  the  bearing  cap  is  not  pulled  down  very 
tightly  it  will  shift  or  “work”  while  the  engine  is  running, 
and  it  is  only  a matter  of  time  until  the  main  bearing  studs 
will  be  crystallized  and  broken. 

When  fitting  new  connecting  rods  or  newly  babbitted  con- 
necting rods  to  a crank  shaft,  it  is  necessary  to  test  them  for 
alignment.  One  method  is  to  put  a bar  through  the  wrist 
pin  end  of  the  rod,  set  the  rod  up  on  the  crank  shaft  and 
measure  from  the  main  journal  on  either  side  up  to  the  ends 
of  this  bar,  to  find  out  whether  the  connecting  rod  stands 
exactly  perpendicular  to  the  crank  shaft.  (Old  connecting 
rods  can  be  used  for  this  purpose.) 

Disassembling  and  Reassembling  Piston  Pin  Bearings. 

Explain  methods  of  avoiding  the  straining  or  distorting  of 
the  pistons  and  also  the  necessity  for  these  precautions. 

Most  of  our  modern  motors  use  the  aluminum  alloy  piston. 
Due  to  the  expansion  of  the  pistons  when  hot,  the  wrist  pins 
must  fit  tightly  while  the  pistons  are  cold.  And  as  we  do  not 
wish  to  distort  the  piston  by  forcing  the  wrist  pin  into  it, 
we  sometimes  heat  the  piston  in  hot  oil  or  water  and  then 
drop  the  wrist  pin  in,  thus  avoiding  some  chances  of  distorting 
the  piston. 

The  piston  rings  should  be  removed  and  replaced.  Explain 
methods  and  precautions.  Explain  that  the  lower  edge  of 
the  piston  ring  and  the  lower  edge  of  the  ring  slot  are  as  im- 
portant as  the  face  of  the  ring;  because  the  rings  must  be  free 
in  the  slots  and  therefore  the  pressure  and  the  hot  gases  will 
get  in  behind  the  rings,  and  unless  the  lower  edge  is  perfect, 


9 

will  get  out  under  the  rings.  In  this  way  rings  are  made 
practically  useless.  Rings  must  be  returned  to  their  own 
slot  in  the  same  position  as  they  were  before  (same  side  up). 

Explain  fitting  of  piston  rings  and  particularly  the  adjusting 
of  the  gap  in  the  ring  to  allow  for  expansion.  Rings  must 
not  be  stretched  or  twisted. 

Grinding  and  Testing  the  Valves. 

Most  mechanics  have  a tendency  to  grind  valves  to  excess. 
It  is  best  to  grind  them  as  little  as  possible.  In  other  words, 
if  the  valve  is  badly  pitted  do  not  try  to  remove  all  the  pits, 
but  grind  until  you  have  a good  width  of  seat  all  the  way 
around  the  valve. 

The  valve  springs  should  be  tested  while  compressing  them 
to  the  length  which  they  occupy  while  on  the  cylinder  with  the 
valve  seated.  Find  out  what  their  tension  is  by  weighing 
them  on  a spring  scale. 

Oil  up  the  cylinder  and  pistons  before  assembling.  Be 
sure  to  explain  that  rags  must  never  be  used  for  this  purpose. 
Use  only  the  bare  hand  which  can  be  easily  freed  from  all 
grit,  and  while  oiling  with  our  hands  we  can  detect  grit  if 
present.  Oil  the  piston  thoroughly,  under  the  rings,  in  the 
wrist  pin  and  all  over  the  outside.  Oil  the  cylinder  all  over 
the  inside ; leave  no  dry  spots.  Explain  precautions  in  slipping 
the  cylinder  over  the  pistons.  If  a ring  is  broken,  it  will  score 
the  cylinder.  See  that  the  cylinder  gasket  is  not  doubled  over 
on  one  corner,  because  there  is  danger  of  throwing  the  cylinder 
out  of  line  in  this  way.  When  engines  have  separate  cylinders, 
it  is  necessary  to  line  them  up  carfully  with  a straight-edge 
so  that  the  inlet  manifold  will  not  have  to  be  sprung,  and  be 
under  strain  when  bolted  to  the  cylinders.  It  is  usual  to  test 
the  manifolds  after  assembling  by  blowing  into  them  by 
mouth  or  with  air  pressure;  then  put  oil  on  all  the  joints  and 
see  if  bubbles  come  through. 

The  water  pump  stuffing  boxes  should  be  carefully  packed. 
Explain  the  importance  of  polishing  the  shaft  before  packing 
to  prevent  excessive  wear  on  the  packing. 

Explain  the  necessity  of  putting  in  the  largest  possible 
amount  of  packing,  because  the  more  packing  you  can  get 
into  the  stuffing  boxes  the  looser  it  can  be  left  without  leaking 
and  less  wear  and  less  friction  will  result. 

Explain  that  the  stuffing  boxes  should  never  be  allowed  to 
leak,  because  if  they  do,  the  shaft  will  rust,  become  rough,  and 
cut  out  the  packing. 


10 

Assemble  the  valve  operating  gear.  In  high  speed  motors, 
it  is  very  essential  to  have  every  part  of  the  valve  operating 
mechanism  working  perfectly  freely.  Adjust  the  valve  clear- 
ance. (See  Lecture  No.  III.)  Time  the  cam  shaft.  (See 
Lecture  No.  III.)  Time  the  magneto  and  wire  up.  (See 
Lecture  No.  V.) 

Be  sure  to  have  every  man  in  the  class  go  through  these 
processes  by  himself  and  be  sure  he  understands  it.  Never 
allow  them  to  call  the  process  complete  without  checking  it 
afterwards  to  find  out  exactly  what  results  they  have  obtained. 
This  is  important,  even  with  the  best  mechanics  in  airplane 
work.  Always  preach  the  idea  of  preventing  trouble  where 
possible. 

All  the  way  through  the  process  of  assembling  a motor, 
bear  in  mind  the  necessity  of  having  an  oil  supply  for  all 
moving  parts  run  for  the  first  few  minutes,  before  the  oil 
pump  can  fill  all  the  passages  and  deliver  oil  to  all  parts. 

If  possible,  these  motors  which  we  are  just  finishing  over- 
hauling should  be  installed  on  a testing  block  or  in  a fuselage, 
and  run.  Then,  if  time  and  material  permit,  widely  different 
types  should  be  overhauled. 

Block  Work. 

Actual  power  determinations  are  of  comparatively  small 
practical  value  to  an  airplane  mechanic,  but  the  motor  can 
be  run  with  a propeller  or  a club.  While  installing  the  motor, 
impress  upon  the  class  that  the  switch  and  ground  wire  must- 
be  connected  before  the  propeller  is  put  on.  In  this  way,  the 
motor  can  be  made  safe  while  the  propeller  is  on,  there- 
by lessening  the  danger  of  some  one  being  kicked.  On 
many  engines  it  is  necessary  to  place  the  propeller  in  relation 
to  the  spark  timing,  so  that  the  propeller  will  come  in  a posi- 
tion which  is  convenient  for  a strong  pull  while  the  magneto  is 
in  a certain  position.  In  case  it  should  be  necessary  to  explain 
this,  it  can  be  done  as  follows : 

Place  the  propeller  on  the  engine  but  without  fastening,  so 
that  it  can  be  swung  freely  without  revolving  the  engine. 
Now  stand  in  the  proper  position  to  crank  the  propeller,  and 
hold  the  blade  in  the  position  where  it  will  be  convenient  to 
start  the  pull.  Note  this  position.  We  must  have  room  to 
pick  up  speed  before  the  spark  occurs,  to  avoid  back  kicks. 
Therefore,  move  the  propeller  down  about  twenty  degrees  and 
note  this  position.  This  is  where  we  wish  the  “break”  or 
spark  to  occur.  Next,  attach  the  propeller  lightly  so  the 


11 

motor  can  be  turned  over.  Open  the  breaker  box  on  the  mag- 
neto, turn  the  propeller  until  a break  occurs  (it  doesn’t  matter 
in  which  cylinder).  Now  disengage  the  propeller;  put  it  in  the 
position  where  we  said  we  wished  the  spark  to  occur;  secure  it 
in  the  proper  manner.  This  simply  brings  the  explosion  in 
the  proper  position  for  cranking. 

Precaution. 

In  most  magnetos,  when  the  breaker  cover  is  removed,  the 
switch  no  longer  short  circuits  the  magneto;  therefore,  while 
performing  this  operation  of  placing  the  propeller,  it  will  be 
wise  to  remove  the  connection  between  the  collector  brush 
and  the  distributor.  Or,  if  this  is  not  accessible,  remove  the 
wires  from  the  distributor  or  from  the  spark  plugs,  because 
there  may  be  enough  gasoline  fumes  in  the  cylinder  to  cause 
an  explosion  and  a “kick.” 

Propeller  Notes. 

Never  rock  the  propeller  as  a preparation  to  the  final  pull 
for  starting.  Many  people  are  hurt  in  this  way.  Place  your 
propeller  where  you  intend  to  start  the  pull,  raise  up  on  tip 
toes,  and  start  your  pull  strongly.  Remember  you  must  pick 
up  a great  deal  of  speed  in  the  first  few  degrees  in  order  to 
“carry  over”  the  spark.  As  you  finish  the  pull  or  stroke, 
manage  so  that  you  will  be  withdrawing  your  hand§.  If  the 
motor  kicks  back,  never  try  to  resist  it;  simply  withdraw  your 
hands  instantly.  Never  have  tools  in  your  pockets  while 
cranking.  They  may  fly  out  of  your  pockets  and  be  “batted” 
through  your  legs  by  the  propeller. 

The  man  at  the  propeller  and  the  man  at  the  switch  should 
“sound  off”  what  they  are  doing,  so  there  may  he  no  misunder- 
standing. Make  it  a rule  that  the  man  at  the  switch  will  not 
say  “closed”  until  the  switch  is  closed,  because  if  he  should  say 
“closed”  and  then  leisurely  reach  over  to  close  the  switch  or 
make  the  motor  safe,  the  man  at  the  propeller  might  work 
faster  and  pull  the  propeller,  believing  the  engine  to  be  safe, 
and  get  a severe  kick. 

It  is  very  important  to  have  the  throttle  closed  or  nearly 
closed  whenever  the  switch  is  open.  On  the  testing  block, 
cranking  the  motor  with  the  throttle  open,  to  start  it,  will 
only  result  in  the  motor  starting  violently  and  a possibility 
of  injuring  the  man  cranking;  but  in  an  airplane,  the  danger  is 
greater  because  the  machine  is  likely  to  start  ahead  and  seri- 


12 

ously  injure  the  man  cranking  the  propeller  as  he  would  not  be 
able  to  get  out  of  the  way. 

After  installing  the  motor,  make  the  class  inspect  the  water, 
oil  and  gas  supply  and  also  the  ground  wire  and  switch.  Start 
the  motor  and  run  it  slowly  at  first.  Point  out  that  the  motor 
must  be  warmed  up  slowly  and  gradually  to  avoid  unequal 
expansion  and  consequent  cracking  of  parts. 

Carburetor  Adjustment  on  the  Block. 

This  work  is  particularly  valuable  in  training  the  pupil’s 
ear.  The  Model  “L”  Schebler  carburetor  is  suitable  for  this 
work,  because  it  is  adjustable  yet  simple.  To  make  the  ad- 
justment of  the  carburetor  as  simple  as  possible,  adjust  the 
auxiliary  air  valve  spring  so  that  it  just  holds  the  valve 
against  the  seat,  before  starting  the  motor.  This  leaves  onty 
the  gasoline  adjustments  to  be  made. 

Then  emphasize  the  fact  that  the  looser  the  auxiliary  air 
valve  spring  is  left,  the  larger  volume  of  air  will  pass  through 
the  inlet  pipes  to  the  cylinders,  giving  full  charges  of  gas  to 
the  cylinders  and  therefore  more  speed.  The  limit  to  this  is, 
that  if  the  air  valve  is  too  loose  the  motor  will  not  be  able  to  get 
enough  gasoline  even  with  the  gasoline  adjustments  wide  open 
and  also  the  motor  will  be  liable  to  stop  when  throttled  down, 
and  “pick  up”  badly. 

Have  every  student  go  through  this  work  himself,  if  gasoline 
and  time  are  available.  Teach  them  that  they  must  learn 
to  make  the  adjustments  quickly  to  avoid  over-heating  of  the 
motor  in  airplane  work.  Then  disarrange  the  adjustments 
after  each  student  finishes. 

It  is  well  to  use  open  exhausts  (no  muffler)  for  this  work, 
so  the  class  will  become  used  to  the  noise  and  also  the  appear- 
ance of  the  exhaust  under  the  various  mixtures  and  conditions. 

Diagnosis  of  Trouble. 

This  is  a very  practical  subject  and  can  be  so  taught  that 
it  will  be  very  valuable  not  only  to  the  mechanic  but  also  the 
pilot.  The  work  consists  of  artificially  causing  troubles  which 
really  do  occur  in  practice  on  the  field,  then  having  the  class 
find  the  troubles.  It  is  necessary  to  have  the  motor,  prefer- 
ably an  airplane  motor,  mounted  on  a block  where  it  can 
be  run.  The  instructor  should  plan  the  troubles  carefully, 
simple  ones  at  first,  and  plan  so  there  will  not  be  two  troubles 
giving  the  same  symptoms  at  the  same  time. 


13 

To  get  the  real  value  out  of  this  work,  first  give  Lecture  No. 
VIII  on  system  for  diagnosis  of  trouble.  Do  not  let  the  class 
go  ahead  and  find  troubles  by  inspection  and  haphazard 
methods,  but  make  them  reason  out  where  the  trouble  is  and 
why.  If  necessary  reason  out  loud  for  them.  Never  make 
troubles  by  changing  carburetor  adjustments  and  never  allow 
the  class  to  correct  troubles  by  changing  these  adjustments. 
By  following  this  rule,  you  will  help  to  combat  a failing  of  the 
human  race,  viz.,  to  try  to  correct  all  troubles  by  adjusting 
the  carburetor. 

The  following  are  some  of  the  troubles  we  make  on  the 
Curtiss  8-cylinder  engines.  They  may  serve  as  a suggestion 
for  this  work. 

Ignition  Troubles. 

A Bosch  magneto  type  DR8  is  suitable  for  this  work. 

Bad  plug. 

Shorted  spark  plug. 

Loosened  spark  plug  gasket. 

Spark  plug  wire  off. 

Spark  plug  wire  shorted  against  motor. 

Ground  wire  short  circuited. 

Magneto  leads  crossed. 

Magneto  leads  disconnected 

Magneto  brushes  missing.  (Collector  brush,  bridge  brush, 
ground  brush,  and  distributor  brush.) 

Broken  insulation  at  spark 

Dirt  in  breaker. 

Various  breaker  troubles.  (Points  of  breaker  apart;  out  of 
adjustment  and  breaker  screw  loose.) 

Magneto  firing  on  the  wrong  stroke. 

Safety  gap  shorted. 

Breaker  timed  wrong.  (Off  the  key.) 

Water  in  magneto. 

Various  shorts  in  the  secondary.  For  example,  drill  a small 
hole  in  the  distributor  arm  to  the  middle  of  the  core  in  the 
center;  insert  a small  wire  in  this  hole  and  cover  over  the 
surface. 

Magneto  fully  retarded  and  key  removed  from  driving  gear 
so  magneto  will  shift  out  of  time. 

Magneto  timed  on  wrong  top  center. 

Carburetor  Troubles. 

(Note. — Schebler  Model  “L”  is  suitable  for  this  work.)  . 

Remove  the  auxiliary  air  valve  spring.  Block  the  auxiliary 
air  valve  open  (with  short  piece  of  copper  tubing). 


14 


Remove  float  valve. 

Partially  obstruct  float  valve. 

Obstruct  spray  nozzle,  partially  and  completely. 

Loosen  joint  in  the  inlet  manifold. 

Put  bad  gaskets  in  the  inlet  manifold  joints.  (Gaskets 
which  will  cause  a leak.) 

Put  water  in  the  float  valve  of  your  carburetor. 

Cause  the  float  to  stick. 

Replace  the  lift  lever  spring  with  a weaker  one,  so  that  the 
roller  will  not  follow  the  cam. 

Plug  the  air  vent  in  the  gas  tank. 


Valve  Troubles. 

No  clearance,  or  valve  held  open. 

Cam  follower  stuck  (no  lock  washer  on  set  screw). 

Defective  push  rod. 

Cam  shaft  gears  meshed  wrong. 

Weak  exhaust  valve  springs. 

Cut  the  points  off  the'  cam-follower-guide-set-screw,  so  it 
will  allow  the  cam  follower  to  turn. 

Piece  of  wire  holding  valve  off  seat. 

Cam  follower  turned  around. 

Water  in  the  cylinder.  Fill  your  cylinder  full  of  water  while 
the  piston  is  at  the  bottom  of  the  compression  stroke,  and  if 
the  water  gets  in  several  of  the  cylinders  on  the  same  side,  have 
the  class  blow  the  water  out  by  removing  the  spark  plugs  on 
that  side  of  the  motor  and  running  the  motor  on  the  other 
four  cylinders.  This  is  an  effective  way  of  removing  water 
from  the  cylinders. 


Inspection  for  Prevention  of  Trouble. 

The  instructor  should  loosen  up  and  disarrange  numerous 
parts  of  the  motor,  propeller  fastenings,  gas  feed,  cooling  and 
lubrication  systems,  making  note  of  all  he  doe§.  Then  call 
the  class  out  and  explain  that  they  are  supposed  to  fly  over  two 
hundred  miles  of  rocky  mountains  with  this  motor  today  and 
it  will  be  necessary  to  inspect  with  the  idea  of  preventing  trouble , 
and  make  note  of  all  they  find  wrong.  When  they  are  through 
compare  the  notes  and  tell  the  class  whether  or  not  they  can 
theoretically  fly  safely  across  the  mountains;  then  have  them 
start  and  run  the  motor. 


15 

Emergency  Repairs. 

Remove  suitable  parts  of  the  motor,  magneto  or  carburetor 
and  have  the  class  make  emergency  repairs  with  iron  wire 
and  tape. 

Have  each  section  select  tools  and  supplies  which  they 
consider  most  necessary  to  take  along  in  an  airplane  on  a long 
trip.  For  example,  tape,  assorted  nuts,  cotters,  etc.,  soft 
wire,  spark  plugs,  exhaust  valve  springs.  Distribute  the  tools 
so  there  will  be  a wrench  for  every  part  of  the  motor,  but  no 
excess  wrenches;  try  to  find  one  wrench  which  will  answer 
many  purposes.  Explain  that  the  most  serious  work  neces- 
sary to  prepare  for  would  be  the  removal  of  one  cylinder. 


TEN  PRACTICAL  LECTURES  ON  AIRPLANE 
MOTORS. 


(Note. — The  lecture  room  should  be  equipped  with  black- 
boards, and  lectures  should  be  illustrated  by  diagrams  on  these 
boards.  Also  a “cut-away”  model  of  a four-cycle  motor, 
showing  all  moving  parts,  magnetos,  carburetors,  any  small 
parts  which  might  help  to  illustrate  the  lectures.) 

LECTURE  I. 

Subjects. — Principle  of  four-stroke  cycle  engine,  heat  loss, 
and  reasons  for  cooling.  (Note. — Nomenclature  of  parts 
should  be  taught  about  this  time,  on  the  model.) 

It  is  of  vital  importance  thoroughly  to  understand  the  ele- 
mentary principles  of  the  motor,  because  once  they  are  mas- 
tered, we  can  reason  out  the  solution  of  many  motor  difficul- 
ties. 

1.  First,  let  us  consider  the  burning  of  city-gas,  in  the  open, 
unconfined.  If  we  light  the  gas,  as  it  flows  out  of  a gas  jet,  it 
burns  with  a small  amount  of  heat  and  without  generating 
any  pressure,  and  with  no  noise.  Now,  if  we  take  a pipe 
several  feet  long,  closed  at  one  end  and  put  in  enough  gas 
to  fill  it  for  six  or  eight  inches,  and  then  ignite  the  gas,  it  will 
burn  and  rush  violently  out  of  the  open  end  of  the  pipe.  If 
we  should  take  the  same  amount  of  gas  and  place  a plug 
in  the  pipe,  the  plug  will  be  forced  out  along  the  pipe  for  a 
certain  distance.  Now,  with  the  same  amount  of  gas,  if  we 
force  the  plug  down,  compress  the  gas  into  § of  its  original 
space  and  then  ignite  it,  we  find  the  plug  will  be  driven  much 
further  and  much  faster  than  before. 

2.  In  the  first  gas  engines  made,  gas  was  drawn  into  the 
cylinders  and  burned  without  being  compressed.  The  result 
was  it  required  a great  deal  of  gas  to  develop  a given  amount 
of  horse-power.  In  other  words,  the  engines  were  very  uneco- 
nomical. Later  it  was  found  that  by  compressing  the  gas 
before  firing  they  were  able  to  obtain  a great  deal  more  power 
with  the  same  amount  of  fuel.  When  gas  is  compressed,  it 

(16) 


17 

will  burn  much  more  rapidly.  Naturally,  in  a motor  running 
at  the  speed  that  our  airplane  motors  do,  gas  must  be  burned 
completely  in  a very  short  space  of  time  in  order  to  do  any 
work.  Now,  we  can  understand  that  for  a motor  to  run,  it 
will  be  necessary,  first  to  draw  gas  into  the  cylinder;  then 
the  gas  must  be  compressed,  then  it  must  be  burned  and  ex- 
panded and  the  burned  gas  cleared  out  of  the  cylinder.  These 
four  things  must  be  done  in  any  gasoline  engine. 

Four-Cycle  Engine. 

3.  Practically  all  engines  now  used  in  airplanes  are  of  four- 
cycle, or  more  correctly  speaking,  four-stroke,  cycle  type. 
This  word  cycle  practically  means  a “program.”  The  com- 
plete “program”  of  events  occurring  in  these  engines  could  be 
said  to  be  composed  of  four  “acts”  or  events.  These  four 
events  are  suction,  compression,  expansion  and  exhaust.  Each 
event  requires  one  movement,  or  stroke  of  the  piston  to  com- 
plete it.  The  first  stroke  will  be  an  outward  stroke  of  the 
piston,  drawing  gas  into  the  cylinder.  This  we  will  call 
the  suction  stroke.  The  next  stroke  will  be  an  inward  stroke 
of  the  piston,  compressing  the  gas  in  the  cylinder.  That  is 
the  compression  stroke.  Then  the  gas  will  be  ignited,  and  as 
it  burns,  it  will  expand  and  drive  the  piston  outward.  This 
is  the  working  or  expansion  stroke.  The  fourth  stroke  is  an 
inward  stroke  of  the  piston.  A valve  will  be  opened  and  the 
piston  will  force  out  all  the  remaining  hot  air  and  smoke  left 
from  the  explosion,  thus  leaving  the  cylinder  clean  for  a new 
charge.  This  is  the  exhaust  stroke. 

Thus  we  see  that  we  have  one  working  stroke  and  then 
three  idle  strokes  before  the  next  working  stroke.  If  our 
engine  has  only  one  cylinder,  it  will  be  necessary  to  have  a 
large  fly-wheel  which  will  be  capable  of  storing  up  energy 
during  the  one  working  stroke  sufficient  to  carry  the  piston 
through  the  three  idle  strokes.  In  an  engine  with  more  than 
one  cylinder,  the  other  cylinders  can  take  the  place  of  the 
fly-wheel.  To  sum  up:  the  four  cycle  motor  requires  four 
strokes  to  complete  the  cycle.  These  strokes  are  first,  suction; 
second,  compression;  third,  expansion;  or  working  stroke; 
and  fourth  scavenging  or  exhaust  stroke. 

4.  When  we  compress  air,  it  becomes  heated,  if  we  compress 
it  rapidly  enough.  For  instance,  if  we  are  pumping  an  auto- 
mobile tire,  the  pump  becomes  warm.  This  is  not  entirely  due 
to  the  friction  of  the  plunger  in  the  pump  but  partly  due  to 
the  work  of  compressing  the  air.  The  same  is  due  when  we 


18 

compress  gas  or  a mixture  of  gasoline  and  air  in  an  engine 
cylinder.  As  we  compress  gas  in  the  cylinder,  it  is  heated 
to  a certain  extent.  Then  we  have  the  heat  of  the  electric 
spark  which  we  use  for  igniting  the  gas.  The  heat  of  the 
electric  spark  together  with  the  heat  of  the  compression  is 
sufficient  to  “ignite”  the  gas  or  start  it  burning.  Then  as  the 
gas  burns  its  temperature  and  pressure  will  rise  very  rapidly. 
If  the  gas  were  burned  in  the  open,  the  temperature  would  go 
quite  high,  but  when  burned  in  a confined  space,  the  pressures 
are  increased  as  the  gas  burns.  The  increase  in  pressure  adds 
to  the  heat  (as  in  the  case  of  the  tire  pump) , as  does  also  the  burn- 
ing of  the  fuel.  Therefore,  we  obtain  a temperature  in  a gas 
engine  cylinder  between  2,000  and  2,500  degrees  Fahrenheit. 
You  can  think  of  this  as  a white-hot  flame. 

Heat  Loss. 

5.  Let  us  imagine  we  have  a cylinder  and  piston  so  arranged 
that  we  may  compress  a charge  of  gas  and  then  lock  the  piston 
so  that  it  cannot  be  moved,  and  let  us  also  imagine  that  there 
is  absolutely  no  leakage.  Now,  if  we  ignite  this  compressed 
gas,  our  temperature  and  pressure  will  instantly  run  up  very 
high.  But  should  we  leave  it  for  five  minutes  before  testing 
the  pressure  in  the  cylinder,  we  should  probably  find  but  few 
ounces  of  pressure.  Now  the  reason  for  this  is  that  the  cool 
cylinder  and  piston  have  absorbed  the  heat  of  the  explosion, 
and  as  the  heat  subsided,  the  pressure  subsided  also.  In 
other  words,  the  hot  gases  have  been  contracted  by  cooling. 
In  other  words,  heat  and  pressure  can  be  considered  as  prac- 
tically the  same  thing  in  a gasoline  .engine. 

6.  Now  we  will  stretch  our  imagination  a little  further.  We 
will  imagine  that  we  have  a cylinder  which  is  not  only  capable 
of  having  gas  compressed  in  it,  and  then  the  piston  locked,  but 
also  can  be  maintained  at  a temperature  in  the  neighborhood 
of  2,500  degrees.  This  temperature,  by  the  way,  is  far  above 
the  melting  point  of  iron.  Now,  if  we  can  compress  gas  and 
fire  it  in  this  cylinder  at  this  temperature,  we  will  find  that  the 
pressure  will  remain  very  high  in  the  cylinder  as  long  as  we 
maintain  the  temperature  of  the  cylinder.  This  is  because 
the  cylinder  and  piston  will  not  cool  the  hot  gases  after  the 
explosion.  The  result  is  there  is  no  radiation  or  loss  of  heat 
from  the  explosion  which  is  commonly  called  “heat  loss.” 

7.  With  the  best  gasoline  motors,  we  make  use  of  only 
about  20  per  cent  of  the  heat  value  of  the  fuel,  or  in  other 
words,  we  lose  | of  the  heat  or  power  that  is  in  the  fuel. 


19 

8.  To  s^iow  where  this  heat  or  power  goes,  suppose  our  fuel 
were  100  per  cent  fuel  value.  About  5 per  cent  of  this  is 
consumed  in  friction,  35  per  cent  is  lost  by  radiation,  mostly  to 
the  water  jacket,  and  40  per  cent  goes  out  of  the  exhaust  valve 
and  is  lost.  This  leaves  only  about  20  per  cent  to  be  delivered 
in  the  form  of  power. 

9.  These  figures  are  only  approximate  but  should  serve  to 
give  an  idea  of  where  all  our  power  or  heat  goes. 

10.  Now,  after  explaining  heat  loss,  it  may  seem  odd  that 
we  find  it  necessary  to  cool  the  motor.  However,  I have 
already  explained  that  the  temperature  of  the  explosion  is 
sufficient  to  melt  an  ordinary  iron  cylinder,  and  long  before  the 
temperature  reached  that  point,  the  piston  would  expand  and 
stick  in  the  cylinder;  also  lubrication  would  be  destroyed 
by  the  burning  of  oil  and  the  babbitted  bearings  would  be 
melted  out.  Obviously  then,  the  motor  must  be  maintained 
at  a temperature  sufficiently  low  to  permit  lubricating  oil 
to  work  properly.  However,  there  is  a limit  to  the  tempera- 
ture at  which  a motor  can  run,  which  is  still  lower  than  the 
limit  set  by  the  burning  of  the  lubricating  oil.  Remember 
that  I have  explained  that  if  we  compress  gas  it  becomes 
heated,  and  also  we  must  remember  that  up  to  a certain 
point  the  more  we  compress  the  gas  before  firing,  the  more 
power  we  obtain  for  a given  amount  of  fuel.  However,  if 
we  compress  the  gas  to  excessive  pressure,  the  heat  of  com- 
pression will  ignite  the  gas,  and  will  ignite  it  long  before  the 
proper  time, — in  other  words,  before  the  piston  nears  the  top 
of  the  stroke, — and  the  result  will  be  a tendency  to  drive  the 
piston  backwards.  Suppose  we  have  an  engine  designed  with 
the  proper  amount  of  compression;  if  some  portion  of  the  cyl- 
inder becomes  red-hot  or  even  approaches  that  temperature, 
the  heat  of  this  portion,  together  with  the  heat  of  compression, 
will  be  sufficient  to  ignite  the  gas — in  other  words,  cause 
what  we  call  “pre-ignition.” 

Cooling. 

11.  There  are  two  methods  used  at  present  for  cooling.  One 
is  by  air-cooling  direct  and  the  other  through  the  medium  of 
water  which  is  later  cooled  by  the  air. 

12.  To  begin  with,  let  us  remember  that  a square  inch  of 
metal  will  radiate  or  give  off  a certain  amount  of  heat  in  a 
given  time.  The  more  square  inches  of  surface  we  have  on 
the  outside  of  a motor  cylinder,  the  more  heat  can  be  radiated 
or  given  off  in  a given  time.  We  are  all  familiar  with  the  ap- 


20 

pearance  of  a motorcycle  cylinder.  It  is  constructed  with 
numerous  fins  or  ridges  around  the  cylinder,  the  purpose  of 
which  is  to  present  as  many  square  inches  of  surface  to  the  air 
as  is  practical. 

13.  Remember  that  the  more  air  that  passes  a square  inch 
of  metal,  in  a given  time,  the  more  heat  the  metal  will  be  able 
to  give  off  or  radiate.  Therefore,  we  find  large  air-cooled 
motors,  arranged  with  a blower  of  some  description  which  will 
cause  a large  amount  of  air  to  flow  by  the  cooled  flanges  on 
the  cylinders,  and  frequently  there  are  jackets  or  housings 
around  the  cylinder  which  will  force  the  air  to  flow  between 
these  cooled  flanges.  This  applies  to  motors  which  have 
stationary  cylinders.  In  the  case  of  rotary  motors,  which  we 
will  take  up  later,  the  cylinders  are  passed  through  the  air 
constantly  and  therefore  no  fan  or  blower  is  required. 

14.  The  other  method  of  cooling  motor  cylinders  is  to  con- 
struct them  with  a double  wall  and  fill  the  space  between  the 
two  walls  with  water.  Then  as  this  water  is  rapidly  heated 
by  the  explosion  inside  the  cylinders,  we  must  have  means  of 
removing  the  hot  water  and  replacing  it  with  cool  water.  For 
a stationary  motor,  we  might  use  a large  tank  of  water  to  draw 
from,  and  pump  the  hot  water  from  the  cylinders  back  into 
this  tank;  but  in  airplanes  and  automobiles  it  is  necessary  to 
carry  a small  amount  of  water  for  the  sake  of  light  weight; 
therefore  we  must  have  a means  of  rapidly  cooling  this  water. 

15.  You  all  know  that  the  way  to  cool  a dish  of  hot  mush 
is  to  spread  it  out  thin,  in  this  way  exposing  many  square 
inches  to  the  air.  If  we  could  carry  a tank  so  arranged  that 
the  water  can  be  spread  out  over  a hundred  square  feet  and 
not  more  than  a sixteenth  of  an  inch  deep,  this  would  make 
an  ideal  radiator  as  far  as  cooling  is  concerned,  but  would  be 
far  too  bulky  and  we  would  lose  water  by  evaporation.  There- 
fore, we  use  radiators  of  various  types.  These  radiators  are 
so  constructed  that  they  spread  the  water  in  very  thin  layers, 
usually  about  the  thickness  of  heavy  paper.  The' 'Water  is 
enclosed  in  thin  sheet  copper,  which  is  an  excellent  conductor 
of  heat.  Water  heats  the  copper  and  the  copper  is  cooled 
by  the  air.  In  a well-designed  radiator,  we  expose  a great 
many  square  feet  of  surface  to  the  cool  air.  In  this  way,  com- 
paratively small  radiators  can  cool  a large  amount  of  water 
in  a short  time.  In  an  airplane,  the  radiator  is  driven  con- 
stantly through  the  air  at  a high  speed,  thus  insuring  an  ample 
supply  of  cool  air. 

16.  There  are  two  methods  of  taking  the  water  from  the 
radiator  to  the  cylinder  and  from  the  cylinder  back  to  the 
radiator,  or  in  other  words,  circulating  the  water.  First,  we 


21 

have  the  system  which  employs  a pump  of  some  type,  usually 
a centrifugal  pump.  This  pump  circulates  the  water  through 
the  water  jakets  at  the  speed  which  is  found  to  be  best  by  the 
designer  of  the  motor.  Also  we  have  what  we  call  the 
Thermo-Syphon  system,  or  in  other  words,  heat  syphon. 

17.  You  all  know  that  the  hottest  air  in  a room  rises  to  the 
ceiling  and  the  cooler  air  settles  in  the  lower  part  of  the  room. 
This  is  due  to  the  difference  in  weight  of  the  hot  and  cold  air. 
The  same  thing  is  true  with  water  in  a tank.  If  you  fill  a 
tank  with  warm  water,  very  soon  you  will  find  comparatively 
cool  water  in  the  bottom  while  the  water  on  the  surface  will 
be  warm. 

18.  In  the  cooling  systems  of  motors,  this  works  out  as 
follows.  The  water  in  the  water  jacket  surrounding  the  cyl- 
inder is  being  heated  and  is  rising.  It  flows  upward  to  the  top 
of  the  radiator,  and  as  it  flows  upward  its  place  is  taken  by  cool 
water  from  the  bottom  of  the  radiator;  also  in  the  radiator,  the 
water  is  being  cooled  and  as  it  cools,  it  becomes  heavier  and 
settles  to  the  bottom.  In  this  way,  we  have  the  heating  of 
the  water  in  the  cylinder  and  the  cooling  of  the  water  in  the 
radiator  causing  the  water  to  circulate  continually.  This 
system  has  the  advantage  of  simplicity  and  also  of  keeping 
the  cylinder  head  comparatively  hot  even  when  the  motor  is 
run  slowly.  However,  it  requires  larger  water  jackets  and 
water  passages,  which  mean  more  weight  and  this  is  probably 
the  reason  it  is  not  used  in  airplane  work.  With  the  pump  or 
force  circulation  systems  we  find  the  designers  taking  advan- 
tage of  this  Thermo-Syphon  principle  in  this  way.  Water 
is  always  introduced  at  the  bottom  of  the  water  jacket  and 
removed  at  the  highest  point  of  the  water  jacket,  delivering 
to  the  top  of  the  radiator  and  drawing  from  the  bottom. 
In  this  way  the  Thermo-Syphon  principle  helps  the  pump 
instead  of  hindering  it. 


' LECTURE  II. 

COMPARISON  OF  AIR  AND  WATER  COOLED  TYPES. 

1.  The  air  cooled  motor  can  run  at  higher  temperatures 
than  the  water  cooled  motor,  and  as  there  is  less  difference  in 
temperature  between  the  cylinder  walls  and  the  heat  of  explo- 
sion, there  is  less  loss  of  heat  by  radiation  or  less  “heat  loss.” 
The  air  cooled  system  is  simpler  and  sometimes  lighter  than 
the  water  cooled.  It  has  no  freezing  troubles  in  winter,  but 


22 


as  it  runs  hotter  this  means  lower  compression  must  be  used, 
because  on  hot  days,  with  the  atmospheric  temperature 
higher,  the  cylinder  temperature  may  be  several  degrees 
hotter.  In  other  words,  the  designer  has  no  definite  upper 
limit  on  his  temperature  and  must  make  the  combustion 
chamber  of  a size  which  will  give  a compression  suited  to  the 
highest  likely  temperature.  This  means  comparatively  low 
compression.  With  water  cooled  motors,  as  the  water  tem- 
perature reaches  212  degrees  (boiling  point  of  water)  the  water 
is  rapidly  lost.  Therefore,  there  is  quite  a definite  upper  limit 
to  the  temperature  at  which  this  type  of  motor  can  be  run, 
and  this  means  that  a comparatively  high  compression  can 
be  used  and  better  fuel  economy  obtained. 

2.  When  a motor  has  excessive  compression,  it  will  run  well 
for  a short  time,  then  will  lose  power  from  pre-ignition.  Air- 
plane motors  are  particularly  likely  to  be  troubled  with  pre- 
ignition, because  of  the  fact  that  they  are  run  continuously 
on  full  throttle,  which  also  means  full  compression. 

Types  of  Motors. 

3.  The  type  of  motors  with  which  we  are  all  familiar  is 
the  vertical,  where  the  cylinders  are  arranged  in  a row,  stand- 
ing vertically  over  the  crank  shaft.  Motors  with  one,  two, 
three,  four  or  six  cylinders  nearly  always  are  made  in  this 
type.  If  we  attempted  to  make  eight  cylinder  motors  in 
this  type  with  the  cylinders  in  line,  it  would  make  a very  long 
motor  which  would  take  too  much  room  in  the  airplane 
fuselage  and  also  the  crank  case  would  have  to  be  made  quite 
heavy  in  order  to  be  stiff  enough.  Eight  and  twelve  cylinder 
engines  can  be  built  with  the  cylinders  in  line,  or  vertical,  for 
marine  purposes,  but  are  nof  likely  to  be  built  in  that  way  for 
airplanes. 

4.  Suppose  we  decide  to  build  an  eight  cylinder  motor. 
Now,  if  we  can  arrange  the  cylinders  in  two  rows  of  four,  side 
by  side,  we  cut  in  half  the  length  and  weight  of  the  crank  shaft, 
cam  shaft  and  crank  case,  thus  making  the  motor  lighter, 
more  compact  and  more  rigid. 

5.  We  have  airplane  motors  built  of  still  another  type, 
where  the  cylinders  are  placed  around  a single  crank  pin  in 
the  same  manner  as  the  spokes  of  a wheel  are  placed  around 
the  hub.  We  call  this  the  radial  type.  This  type  gives  a very 
short  and  rigid  shaft.  The  crank  case  also  is  a strong  barrel 
type  of  case  not  divided  in  the  center.  Also  it  is  possible 
with  a crank  shaft  like  this  to  use  ball  bearings  satisfactorily. 


23 

6.  This  type  of  motor  is  built  both  in  the  air  and  water 
cooled  types.  In  the  air-cooled  form  it  requires  no  fan  or 
blower,  because  the  cylinders  are  so  placed  that  the  air  will 
reach  all  of  them  evenly  without  any  blower  or  jacket.  There 
is  a tendency  for  the  lower  cylinders  to  become  flooded  with 
oil,  but  this  has  been  satisfactorily  overcome  by  constantly 
pumping  the  oil  out  of  the  crank  case  and  not  allowing  it  to 
accumulate  there.  This  type  of  motor  is  hard  to  enclose  in  a 
stream-line  hood  and  therefore  has  not  been  used  on  high  speed 
machines. 

7.  We  have  still  another  type  of  motor  built  with  the  cylin- 
ders arranged  in  the  same  manner  as  radial  motors  but  oper- 
ating differently.  Up  to  the  present,  we  have  considered  only 
stationary  cylinder  types  in  which  the  cylinders  remain 
stationary  and  kick  or  drive  the  crank  shaft  around.  Others 
are  known  as  rotary  or,  correctly  speaking,  rotating  motors. 
The  cylinders  and  crank  case  are  similar  in  arrangement  to  the 
radial  type. 

8.  In  the  rotary  type  of  engine,  the  crank  shaft  is  bolted  to 
the  airplane  and  the  propeller  is  attached  to  the  crank  case, 
just  the  reverse  of  the  practice  with  stationary  cylinder  motors. 
The  crank  shaft  then  stands  still  and  the  cylinders  drive 
themselves  around  it.  With  this  type  of  motor,  we  have 
to  deal  with  centrifugal  force.  In  other  words,  there  is  a 
strong  tendency  for  the  cylinders  to  be  thrown  away  from  the 
center  of  the  engine.  This  makes  a steel  construction  neces- 
sary, and  in  order  to  reduce  this  strain  as  much  as  possible, 
the  cylinders  are  built  of  steel  with  very  thin  walls  of  very  light 
construction. 

9.  The  rotary  engines  combining  the  principles  which  tend 
to  lighten  the  weight,  as  in  radial  motors,  with  an  extremely 
light  all-steel  construction,  are  the  lightest  motors  now  in  use. 
But,  unfortunately,  they  consume  so  much  fuel  and  oil  that 
if  we  weigh  them  with  fuel  for  a long  flight,  they  are  heavier 
that  the  stationary  cylinder  types;  but  for  short  flights,  they 
remain  the  lightest.  The  reasons  for  their  excessive  fuel 
consumption  are,  first,  that  as  they  are  air  cooled  they  have 
comparatively  low  compression;  and  also  that  they  require 
between  ten  and  twelve  per  cent  of  the  power  they  develop  to 
rotate  the  engine,  or  drive  the  cylinders  through  the  air. 

Types  of  Cylinders. 

10.  There  are  two  main  types  of  cylinders  used.  They 
differ  in  location  and  operation  of  their  valves,  and  also  in  the 
form  of  their  combustion  chambers.  The  “L”  head  type  has 


24 

the  valves  side  by  side  in  a pocket  at  one  side  of  the  combus- 
tion chamber.  The  valves  are  operated  by  simple  cam  fol- 
lowers or  plugs  riding  upon  the  cam  shaft  which  is  in  the  crank 
case.  The  cams  raise  the  followers  which  raise  the  valves, 
making  a simple  valve  gear.  This  design  makes  room  for 
long  valve  stem  guides  and  springs,  and  there  is  a straight 
thrust  on  the  valve  stem  and  no  side  thrust  which  tends  to 
wear  the  guides. 

11.  The  valve  in  the  head  type  of  cylinder  has  the  valves 
placed  either  vertically  or  at  a slight  angle  apart,  with  the 
heads  of  the  valve  seating  in  the  head  of  the  cylinder.  This 
gives  a combustion  chamber  that  has  no  pockets,  but  the  valve 
must  be  operated  by  push  rods  and  rocker  arms  or  else  the 
cam  shaft  must  be  placed  along  the  top  of  the  cylinder  with 
small  rocker  arms  operating  the  valve  direct  from  the  cam. 
In  this  case  the  cam  shaft  is  held  in  an  oil-tight  housing. 

12.  Now  I will  ask  you  to  recall  the  explanation  of  heat  loss 
in  the  first  lecture.  You  will  remember  that  a large  amount 
of  heat  was  lost  by  radiation,  or  by  the  interior  surfaces  of  the 
combustion  chamber  absorbing  heat.  The  more  square  inches 
of  surface  exposed  to  the  heat  in  the  combustion  chamber,  the 
more  heat  will  be  absorbed  and  the  greater  will  be  the  heat 
loss.  Evidently  then,  the  combustion  chamber  which  exposes 
the  least  surface  to  the  heat  will  cause  the  least  heat  loss.  This 
also  works  another  way;  the  cylinder  exposing  the  least  sur- 
face to  the  heat  of  combustion  in  the  combustion  chamber 
will  require  the  least  cooling.  This  may  mean  reduced  weight 
in  the  cooling  system.  In  the  case  of  high  speed  motors,  it  is 
extremely  important  to  have  very  direct  gas  passages.  In 
other  words,  the  gas  should  be  able  to  go  straight  into  the 
cylinder  and  straight  out  without  having  to  flow  around  any 
corners  or  through  pockets  in  the  side  of  the  cylinders.  The 
“valve-in-the-head”  type  of  cylinder  gives  a more  direct  gas 
passage,  which  means  such  a cylinder  can  receive  a more  com- 
plete charge  of  gas  at  high  speed  and  the  exhaust  gases  can  be 
more  completely  expelled  or  scavenged. 

Spark  Timing  or  Spark  Advance. 

13.  Up  to  the  present,  we  have  considered  ignition  of  gas  as 
occurring  when  the  piston  is  at  the  top  of  its  stroke.  It  is 
important  to  ignite  the  gas  or  start  it  burning  at  such  a time 
that  it  will  be  completely  burned  and  ready  to  do  work  by  the 
time  the  piston  starts  down.  If  our  motor  is  run  at  an  ex- 
tremely slow  speed  we  can  ignite  the  gas  when  the  piston  is  at 


25 

the  top  of  its  stroke,  and  as  the  combustion  or  burning  of  the 
fuel  is  quite  rapid,  the  fuel  would  be  completely  burned  by  the 
time  the  piston  starts  down.  But,  if  the  motor  is  run  at  high 
speed,  the  crank  shaft  will  turn  many  degrees  during  the  time 
that  the  gas  is  burning  and  if  we  ignite  the  fuel  when  the  piston 
is  already  at  the  top  of  the  stroke,  the  gas  will  not  be  completely 
burned  and  the  maximum  pressure  will  not  be  obtained  until 
the  piston  is  already  very  far  down  on  the  working  stroke. 
Therefore  we  will  not  get  all  the  power  from  this  explosion. 

14.  In  high  speed  motors,  the  spark  must  occur  far  enough 
before  the  piston  reaches  the  top  of  its  stroke  so  that  the  fuel 
will  be  completely  burned  and  the  maximum  pressure  and  heat 
obtained  by  the  time  that  the  piston  starts  down.  Therefore, 
the  faster  our  motor  runs  the  earlier  the  spark  must  occur;  also, 
it  makes  considerable  difference  to  the  spark  timing  if  we  use 
different  shaped  combustion  chambers. 

15.  For  example,  if  we  use  the  “L”  type  of  cylinder  with 
one  spark  plug  at  one  side,  the  flame  will  have  a long  way  to 
travel  to  pass  through  the  entire  charge  of  gas.  This  means 
that  it  requires  a long  time  for  the  gas  to  be  completely  burned 
and  such  a type  of  cylinder  will  require  an  earlier  spark,  or 
more  spark  advance,  as  we  say. 

16.  It  is  desirable,  then,  to  use  the  most  compact  combus- 
tion chamber  we  can  and  we  frequently  use  the  two  spark  plugs 
in  each  combustion  chamber  as  far  apart  as  possible.  In  this 
case,  lighting  the  gas  at  two  points,  it  requires  less  time  for 
complete  combustion. 

17.  The  “ valve-in-the-head  ” type  of  cylinder  has  the  most 
compact  combustion  chamber  of  any  type  at  present.  I wish 
to  state  here  that  when  we  crank  our  motor  by  hand  we  are 
turning  it  over  at  a very  low  speed  and  therefore  we  set  the 
spark  to  occur  very  late,  frequently  after  the  piston  has 
already  started  down,  because,  if  we  did  not,  it  would  be  possi- 
ble for  the  explosion  to  drive  the  piston  backwards,  because  of 
its  slow  movement.  This  is  known  as  a “ back-kick”  and  is 
always  dangerous  to  the  man  cranking  the  motor. 

Multi-Cylinder  Engines. 

18.  You  will  remember  that  with  the  single  cylinder  engine, 
we  found  that  we  had  one  working  stroke  followed  by  three 
idle  strokes,  and  a fly-wheel  was  necessary  in  order  to  carry 
the  engine  through  the  three  idle  strokes.  With  this  engine, 
our  power  comes  in  jerks.  There  is  not  a steady  flow  of  power 
from  the  engine,  and,  if  there  is  a heavy  piston  and  connecting 


26 

rod,  moving  up  and  down,  with  nothing  to  balance  it,  the  motor 
causes  considerable  vibration.  While  this  type  of  motor  can 
be  balanced  with  a counterweight  on  the  crank  shaft,  it  never 
works  very  smoothly. 

19.  Now,  if  we  build  a four-cylinder  engine,  we  will  arrange 
the  cylinders  to  fire  once  for  every  stroke  of  the  piston.  In 
this  case,  there  is  a much  steadier  flow  of  power,  or  in  other 
words,  a much  more  even  torque,  and  it  is  usually  arranged 
so  that  there  are  two  pistons  moving  upwards  and  two  moving 
downwards  at  the  same  time,  thus  partially  balancing  the 
engine. 

20.  With  the  six-cylinder  engine,  we  have  still  more  even 
torque  and  better  balance,  and  with  the  eight  or  twelve  cylin- 
der engine,  we  have  a very  steady  flow  of  power,  and  compara- 
tively good  balance  and  smooth  running. 

21.  Now  the  advantages  of  a multi-cylinder  motor  over  a 
motor  with  fewer  cylinders  are  as  follows:  First,  each  piston 
and  connecting  rod  is  smaller  and  therefore  lighter.  These 
parts  are  called  reciprocating  parts  because  of  their  back-and- 
forth  motion,  and  it  requires  a great  deal  of  power  to  start  and 
stop  these  parts  in  a high-speed  motor.  Therefore,  it  is  essen- 
tial that  they  be  as  light  as  possible.  When  we  have  few  cyl- 
inders of  large  diameter  we  have  much  more  difficulty  due  to 
unequal  expansion.  With  smaller  cylinders  there  is  less  of 
this  difficulty.  With  a multi-cylinder  engine  we  have  little 
loss  of  speed  and  power  when  olie  cylinder  fails  for  any  reason. 

22.  In  all  multi-cylinder  engines  all  the  cylinders  fire  during 
one  complete  cycle  (2  revolutions)  and  nearly  all  of  them  are 
arranged  so  that  the  cylinders  fire  at  even  intervals. 

23.  For  example,  a four-cylinder  engine  fires  four  times  in 
two  revolutions,  or  twice  in  one  revolution,  which  means  the 
cylinders  fire  one-half  a revolution  or  180  degrees  apart.  A 
six  cylinder  engine  fires  every  120  degrees;  an  eight  cylinder, 
every  90  degrees  and  a 12  cylinder  every  60  degrees. 

24.  The  length  of  the  working  stroke  is  from  the  top  center 
to  the  time  the  exhaust  valve  opens.  Let  us  say  for  example, 
135  degrees.  If  the  four  cylinder  engine  fires  every  180 
degrees,  there  is  an  interval  of  180  degrees  minus  135  degrees, 
or  45  degrees  when  there  is  no  pressure  to  turn  the  crank  shaft 
ahead  and  the  fly  wheel  must  do  it. 

25.  In  a six  cylinder  engine,  the  cylinders  fire  every  120 
degrees,  which  is  less  than  the  length  of  the  working  stroke,  so 
the  working  strokes  overlap  15  degrees.  This  means  smooth 
running.  In  an  eight  or  twelve  cylinder  engine  the  working 
strokes  overlap  still  more. 


27 

26.  In  all  high-speed  airplane  motors,  it  is  quite  a problem 
to  cool  the  piston  head.  In  fact  it  is  usually  this  part  of  the 
engine  which  limits  the  amount  of  compression  we  can  use. 
The  piston  head  is  cooled  mainly  by  the  heat  flowing  through 
its  metal  to  the  comparatively  cool  cylinder  wall.  If  the 
piston  is  large,  the  heat  has  further  to  travel  from  the  center 
to  the  sides,  and  it  is  quite  difficult  to  keep  this  piston  head 
below  the  temperature  at  which  it  would  ignite  a charge  of 
gas  under  compression.  The  only  way  we  can  use  very  large 
sized  pistons  is  to  reduce  the  amount  of  compression.  By  so 
doing,  the  charge  is  less  easily  ignited  by  hot  parts  and  less 
heat  will  be  generated  with  the  low  compression,  but  it  means 
less  fuel  economy. 

27.  The  advent  of  the  aluminum  alloy  piston  has  been  a 
help  in  the  solving  of  this  problem  because  aluminum  con- 
ducts heat  about  three  times  as  fast  as  iron  or  steel,  which 
were  formerly  used  for  pistons. 

28.  In  practice,  the  multi-cylinder,  small  bore  engines  are 
not  necessarily  more  efficient  or  economical  than  the  larger 
bore  engines  because  they  expose  more  surface  to  absorb  heat 
in  proportion  to  the  cubic  feet  of  gas  handled.  Also  there  is 
more  friction  in  the  multi-cylinder  motor. 

Lubrication. 

29.  Lubrication  consists  in  introducing  some  substance,  for 
example,  oil,  between  two  rubbing  surfaces  to  reduce  the  fric- 
tion and  wear  that  otherwise  would  occur.  No  matter  how 
smooth  a metal  surface  may  appear  to  sight  and  to  touch,  it 
is  in  reality  covered  with  tiny  ridges  and  hollows  that  are 
easily  seen  under  a microscope.  So  when  two  clean,  smooth 
metal  surfaces  are  placed  together  and  caused  to  slide  over 
each  other,  these  little  ridges  engage  each  other,  or  interlock, 
and  naturally  some  of  the  projections  are  torn  loose  from  each 
piece.  This  tearing  away  of  metal  is  known  as  wear,  and  the 
resistance  to  the  tearing  is  known  as  friction.  When  oil  or 
grease  is  put  between  the  two  surfaces,  it  fills  the  little  hollows 
and  forms  a thin  film  or  layer  that  prevents  the  metal  sur- 
faces from  actually  touching  each  other  except  at  the  highest 
points  of  the  largest  of  these  microscopic  ridges.  The  result 
is  a smaller  number  of  these  ridges  torn  loose;  therefore,  less 
wear,  less  friction  and  less  heat  generated. 

30.  This  oil  film  must  be  maintained  in  bearings.  If  it  is 
not,  excessive  wear  always  results.  In  large,  heavy  duty 
machinery,  it  was  found  that  the  oil  film  could  be  maintained 


28 

by  making  the  bearings  large  in  area  so  that  the  pressure  per 
square  inch  would  not  exceed  a certain  point,  beyond  which 
the  oil  film  would  be  destroyed. 

31.  In  airplane  motors,  it  is  impossible  to  use  bearings  of 
this  size  on  account  of  weight.  Therefore,  it  has  been  found 
possible  to  maintain  the  oil  film  by  the  use  of  a large  volume 
of  oil  pumped  through  the  bearings  under  very  high  pressure ; 
but  space  must  be  left  in  the  bearings  for  this  oil  film,  and  also 
it  is  necessary  to  leave  space  and  clearance  in  the  bearings  so 
that  oil  can  be  pumped  through. 

32.  You  can  easily  imagine  that  a heavy  film  of  oil  could  not 
be  forced  through  a bearing  which  had  only  a thousandth  of 
an  inch  clearance.  Motors  using  these  high  pressure  systems 
frequently  have  the  bearings  from  two  thousandths  to  four 
thousandths  of  an  inch  loose,  thus  allowing  the  crank  shaft  to 
run  on  an  oil  film  instead  of  on  babbit  metal.  This  means  less 
friction  and  less  wear  on  the  bearings,  but  it  requires  a high 
oil  pressure. 

33.  I can  now  point  out  that  in  deciding  how  tight  to  adjust 
the  bearings  of  a motor,  we  must  first  consider  the  nature  of 
the  oil  system.  If  we  have  a high  oil  pressure  system,  we  can 
fit  the  bearings  quite  loosely.  In  fact,  we  must  fit  them  loosely 
or  excessive  wear  and  excessive  oil  pressure  will  result.  But 
should  we  have  a motor  using  a splash  system,  gravity  feed 
system  or  very  low  pressure,  we  could  not  leave  the  bearings 
loose  because,  if  we  did,  the  bearings  would  hammer  out 
quickly.  With  such  lubrication  systems,  we  must  adjust  the 
bearings  closer. 

34.  Another  thing  to  be  considered  before  adjusting  the 
bearings  is  whether  we  are  simply  adjusting  bearings  which 
have  been  run  and  are  worn  to  a perfect  bearing  surface  or 
whether  the  bearings  in  question  have  been  newly  scraped 
or  fitted,  and  have  not  worn  to  a perfect  bearing  surface. 
In  the  case  of  the  worn  bearings,  they  will  not  loosen  up  rap- 
idly, and  therefore  cannot  be  adjusted  as  tightly  as  the  bear- 
ings which  have  been  scraped,  have  not  a perfect  bearing 
surface,  and  will  therefore  loosen  up  rapidly  at  first. 

35.  Nearly  all  of  our  modern  airplane  motors  use  this  high 
oil  pressure  system,  generally  having  a gear  pump  in  the  crank 
case  delivering  oil  to  some  tube  running  the  full  length  of  the 
crank  case  which  acts  as  an  oil  distributor.  Sometimes  the 
cam  shaft  is  used  for  this  purpose.  In  any  case,  we  find  oil 
ducts  leading  from  this  tube  to  the  main  bearings  of  the  crank 
shaft,  and  here  we  must  see  to  it  that  the  oil  holes  in  these 
bearings  register  with  the  oil  holes  in  the  crank  shaft,  as  oil  is 


29 

usually  expected  to  flow  into  the  crank  shaft  through  the 
arms  of  the  crank,  out  of  the  crank  pins  and  connecting  rod 
bearings.  The  oil  is  thrown  from  the  connecting  rod  bear- 
ings  up  on  the  piston  and  cylinder  walls  and  also  the  wrist  pins 
or  piston  pins.  The  older  motors  always  had  dip  pans  or 
splash  pans,  arranged  under  the  crank  shaft  in  such  a way  that 
the  connecting  rods  could  dip  into  the  oil  and  splash  it  to  all 
parts  of  the  motor,  but  as  the  airplane  motor  operates  in  all 
positions,  such  systems  proved  unsatisfactory,  because,  when 
in  some  positions,  the  oil  from  these  splash  pans  would  flood 
the  cylinders  and  combustion  chambers,  causing  various 
troubles,  such  as  foul  spark  plugs  and  carbon  deposits. 

36.  Our  modern  engines  have  no  splash  pans  and  have  a 
pump  which  will  keep  all  excess  oil  drained  from  the  crank 
chamber  and  usually  have  a small  oil  sump  under  the  crank 
case  but  not  connected  to  it  in  such  a way  as  to  allow  the  oil  to 
flow  up  into  it.  In  other  words,  the  oil  is  pumped  into  the 
lubrication  system  and  the  crank  case  is  constantly  pumped 
dry  so  that  there  will  be  no  accumulation  of  oil  in  the  crank 
case,  which  might  flood  the  cylinders  in  the  case  of  the  motor 
operating  in  extreme  positions. 

37.  Oils  used  in  motors  are  usually  the  mineral  oils.  Castor 
oil  has  been  used  in  some  types  of  motors  where  the  oil  is  not 

\ used  over  and  over  again.  Castor  oil,  if  used  over  and  over, 
will  congeal  or  thicken.  It  is  useful  in  rotary  motors  because 
it  is  not  mixed  with  gasoline  in  crank  case  and  because 
of  its  high  viscosity  or  ability  to  cling  to  the  cylinder  walls 
under  high  temperature  and  under  the  influence  of  centrifugal 
force  encountered  in  that  type  of  motor.  It  is  finally  thrown 
out  of  the  exhaust  valves  and  lost,  which  accounts  for  the  high 
oil  consumption  of  this  type.  The  desirable  qualities  for 
motor  oil  are  first  of  all  that  it  should  burn  only  at  very  high 
temperatures.  In  other  words,  it  is  said  to  have  a high  fire 
test.  This  is  necessary  to  stand  the  high  temperatures  in 
the  motor  cylinders.  Next,  it  should  maintain  its  viscosity  or 
body  at  high  temperatures  and  after  using,  it  should  show  very 
small  percentage  of  decomposition  so  that  it  will  be  suitable 
for  use  over  and  over  again  in  the  motor.  Also  it  should  have 
a low  carbon  content  so  that  it  will  not  leave  excessive  depos- 
its of  carbon  in  the  combustion  chamber.  As  a rule,  a heavier 
oil  deposits  more  carbon  than  the  lighter  oils.  Lighter  oils 
must  be  used  in  very  cold  climates  because  heavier  oils  become 
too  thick,  and  in  tropical  climates  the  heavier  oils  must  be  used 
as  the  lighter  oils  fail  to  retain  sufficient  body  under  high  tem- 
peratures. 


30 

38.  The  oil  in  a motor  must  be  changed  from  time  to  time, 
that  is,  all  the  old  oil  from  the  lubricating  system  must  be 
removed  and  new  oil  must  replace  it.  Some  of  the  reasons  for 
this  are,  first,  that  the  oil  in  being  thrown  from  the  crank  pin 
or  connecting  rod  bearings  comes  in  contact  with  the  under 
side  of  the  piston  head  and  becomes  partly  burned.  In  this 
way,  it  is  turned  black  and  has  a large  amount  of  carbon  in  it. 
Also  it  becomes  full  of  metal  particles  from  the  wear  of  the 
bearings;  and  still  another  reason  more  recently  discovered  is 
that  when  we  are  using  low  grade  fuels  in  our  motors,  a cer- 
tain proportion  of  this  fuel  fails  to  burn  and  mixes  with  the 
lubricating  oil,  thus  thinning  the  oil  and  impairing  its  lubri- 
cating qualities.  In  other  words,  it  is  poor  economy  to  try 
to  use  oil  too  long. 


LECTURE  III. 

VALVE  TIMING. 

1 . The  valves  of  a motor  must  open  and  close  at  exactly  the 
right  time  in  order  to  let  the  new  gas  in  and  the  burned  gas 
out  of  the  cylinder  at  the  right  time ; also  it  is  important  to  let 
a good  f fully  charge  of  gas  in  and  to  scavenge  or  clean  out  the 
burned  gas  as  completely  as  possible. 

2.  Beginning  with  the  working  stroke:  at  the  beginning  of 
the  stroke,  the  pressure  is  often  between  three  hundred  and 
four  hundred  pounds  to  the  square  inch  (approximately  four 
times  the  pressure  of  compression).  As  the  piston  is  forced 
down  in  the  cylinder,  the  gas  expands  and  pressure  becomes 
less  and  less. 

3.  Finally  a point  is  reached  where  the  pressure  is  down  to 
fifty  or  sixty  pounds  per  square  inch  and  the  crank  is  at  an 
angle  where  the  pressure  has  but  little  effect  on  it,  and  there 
is  not  much  to  gain  by  keeping  it  in  the  cylinder  any  longer. 
There  is  also  a large  amount  of  cylinder  wall  exposed,  to  absorb 
heat;  and  if  the  gas  is  kept  in  the  cylinder  over-heating  will 
result.  Therefore  it  is  usual  to  open  the  exhaust  valve  some 
distance  before  the  piston  reaches  the  bottom  of  its  stroke 
or  the  crank  reaches  “bottom  center.” 

4.  There  is  another  reason  for  opening  the  exhaust  valve 
before  bottom  center.  We  wish  all  the  pressure  in  the  cylin- 
der to  rush  out  before  the  piston  starts  up,  because,  if  it  did 
not,  the  piston  would  have  to  be  forced  up  against  it  and  a 
loss  of  power  would  result. 


31 

5.  The  exhaust  valve  is  kept  open  all  the  way  up  the  ex- 
haust stroke  and  slightly  after  the  top  center  so  the  piston  can 
push  out  as  much  of  the  burned  gas  as  possible.  If  it  is  held 
open  too  long,  the  piston  will  draw  back  some  of  the  burned 
gas  as  it  is  started  down,  and  if  closed  too  soon,  the  cylinder 
will  not  be  completely  scavenged.  So  we  see  that  the  clos- 
ing of  the  exhaust  valve  must  be  quite  accurately  timed.  The 
inlet  valve  usually  opens  at  the  same  time  the  exhaust  valve 
closes  or  a few  degrees  later,  because  we  wish  to  use  the  full 
length  of  the  suction  stroke. 

6.  The  piston,  goes  down  very  rapidly  in  a high  speed  motor 
and  there  will  not  be  time  for  a sufficient  amount  of  mixture 
to  pass  through  the  inlet  valve  to  give  the  cylinder  a full 
charge.  In  other  words,  there  will  still  be  suction,  or  a vacu- 
um, in  the  cylinder  at  the  end  of  this  stroke.  Therefore,  the 
inlet  valve  is  held  open  for  a considerable  period  after  the  pis- 
ton reaches  the  end  of  its  stroke  and  starts  up  on  the  compres- 
sion stroke.  Although  the  piston  is  travelling  upwards,  gas 
will  continue  to  flow  into  the  cylinder  on  account  of  the  suc- 
tion remaining. 

7.  There  is  another  reason  why  the  gas  will  continue  to  flow 
into  the  cylinder.  There  is  a large  column  of  gas  in  the  inlet 
manifold  which  has  considerable  weight,  and  during  the  down- 
ward stroke  of  the  piston,  this  column  of  gas  attains  consider- 
able momentum  and  this  momentum  will  continue  to  force 
gas  into  the  cylinder,  even  after  there  is  no  more  suction  in  the 
cylinder.  So  we  see  why  a motor  will  get  a more  complete 
charge  of  gas  if  the  inlet  is  held  open  some  distance  after  bot- 
tom center.  (Note. — The  faster  the  motor  runs,  the  longer 
the  valve  is  held  open.) 

8.  The  exhaust  valve  opens  from  forty  to  fifty-five  degrees 
before  bottom  center  on  different  motors.  (The  crank  can 
be  set  to  this  angle  by  using  a protractor,  and  the  piston  posi- 
tion measured  while  the  crank  is  in  this  position,  so  we  will 
know  the  correct  piston  position  when  we  come  to  time  the 
valves.  It  is  usually  more  convenient  to  time  airplane  engines 
by  piston  position  than  by  degrees  or  crank  angle  because  they 
use  no  fly  wheel.) 

9.  The  exhaust  valve  closes  on  top  center  or  within  fifteen 
degrees  after  top  center,  varying  on  different  motors.  The 
inlet  valve  opens  at  the  same  time  the  exhaust  valve  closes 
or  possibly  five  or  ten  degrees  later.  (In  a few  engines,  the 
inlet  valve  opens  a few  degrees  before  the  exhaust  valve  closes.) 
The  inlet  valve  closes  from  thirty  to  fifty  degrees  past  bottom 
center. 


32 

10.  The  above  timing  refers  to  all  stationary  cylinder  types 
of  four-cycle  engines  now  used  in  airplanes,  but  is  not  correct 
for  the  Gnome  rotary  engine. 

Method  of  Timing  Valves. 

11.  The  process  of  timing  the  cam  shaft  in  a motor  consists 
in,  first,  placing  the  crank  and  piston  of  one  cylinder  in  the 
correct  position  for  the  exhaust  valve  to  close.  (Note. — We 
always  time  the  crank  shaft  by  the  closing  of  the  exhaust  valve 
because  this  must  be  more  accurately  located  than  any  other 
operation  that  the  cam  shaft  performs.) 

12.  Second,  we  turn  the  cam  shaft  separately  in  the  direc- 
tion in  which  it  runs  until  it  just  allows  the  exhaust  valve 
to  close.  Third,  we  mesh  the  gears  or  connect  the  two  shafts 
together.  If  we  remember  this  simple  explanation,  we  are 
not  likely  to  go  wrong  on  the  valve  timing. 

13.  The  exact  method  of  meshing  gears  or  of  placing  the 
crank  shaft  and  the  cam  shaft  will  vary  somewhat  in  different 
motors,  but  the  principle  remains  the  same. 

14.  It  is  very  important  to  adjust  the  valve  clearances  before 
timing  the  cam  shaft,  because  a small  variation  in  clearance 
makes  a large  variation  in  valve  timing.  If  we  should  time 
the  cam  shaft  without  previously  adjusting  the  clearance,  then 
adjust  the  clearance  afterwards,  this  would  throw  our  valve 
timing  out  or  make  it  incorrect. 

15.  Now  I will  explain  in  detail  the  process  of  timing  the 
cam  shaft  on  a Curtiss  0X2  Motor,  eight-cylinder,  “V”  type. 
First,  we  will  adjust  the  valve  clearance.  At  the  present  time, 
these  clearances  are  supposed  to  be  ten  thousandths  of  an 
inch  on  each  valve.  Then  we  will  remove  the  cam-shaft-gear- 
retaining-screw  and  pull  the  gear  partly  off  the  cam  shaft,  far 
enough  so  that  it  no  longer  engages  with  the  crank  shaft  gear 
but  still  engages  the  key  on  the  cam  shaft.  This  makes  it 
possible  to  turn  either  shaft  independently.  Now  we  will 
remove  the  spark  plug  from  No.  1 cylinder  and  insert  a rod 
or  scale,  and  turn  the  crank  shaft  in  the  direction  of  rotation 
until  we  place  the  piston  exactly  on  top  center.  We  now  have 
a point  to  measure  from.  The  instruction  book  for  this 
motor  will  tell  us  that  the  exhaust  valve  should  close  one 
thirty-second  of  an  inch  past  the  top  center.  Therefore,  we 
will  make  a mark  on  our  rod  or  scale,  just  even  with  the  top  of 
the  spark  plug  hole,  and  will  measure  one  thirty-second  of 
an  inch  up  from  this  point  and  make  another  mark.  As  the 
book  told  us  the  valve  should  close  one  thirty-second  of  an 


33 

inch  after  top  center  we  will  turn  the  crank  ahead  until  the 
piston  has  gone  down  one  thirty-second  of  an  inch.  We  can 
now  see  that  the  crank  shaft  and  piston  in  No.  1 cylinder  are 
in  the  correct  position  for  the  exhaust  valve  to  close. 

16.  Second,  we  will  turn  the  cam  shaft  in  the  direction  m 
which  the  cam  shaft  revolves  until  the  exhaust  valve  is  open, 
and  keep  on  turning  until  the  exhaust  valve  is  just  seated . By 
this  I mean:  not  until  we  have  full  clearance  at  the  exhaust 
valve,  but  just  until  the  tappet-screw  in  the  rocker  arm  is  just 
leaving  the  end  of  the  valve  stem.  Another  method  of  deter- 
mining this,  is  previously  to  insert  a cigarette  paper  between 
the  tappet-screw  and  the  valve  stem  and  when  the  cam  shaft 
is  moved  far  enough  to  allow  this  paper  to  be  slipped  out  with- 
out tearing,  that  will  indicate  that  the  valve  is  just  seated. 
(Note. — A cigarette  paper  runs  one  one-thousandth  to  two 
one-thousandths  of  an  inch  in  thickness.) 

17.  Now  we  can  see  that  our  cam  shaft  is  in  the  position 
where  it  is  allowing  the  exhaust  valve  to  close,  or  that  the 
exhaust  valve  is  just  closed.  Therefore,  we  are  ready  for  the 
third  step,  or  meshing  the  gears.  All  that  is  necessary  to  do 
this,  is  to  push  the  gear  on  to  the  shaft,  but  it  is  possible  that 
after  we  have  placed  both  our  shafts,  the  gear  teeth  may  not 
be  in  position  to  mesh.  In  this  case,  we  will  have  to  decide 
whether  we  prefer  to  shift  the  cam  shaft  a fraction  of  a tooth 
forward  or  backward.  Probably,  shifting  it  backward  will  be 
safer  because  it  is  important  that  we  do  not  allow  the  exhaust 
valve  to  close  before  top  center.  Then,  having  meshed  the 
gears,  the  valve  timing  will  be  correct  for  the  entire  engine,  be- 
cause the  cams  are  so  spaced  on  the  cam  shaft  that  having 
timed  one  cam  correctly  the  rest  will  follow  in  proper  order. 
In  airplane  work,  when  we  are  timing  the  cam  shaft  or  the 
magneto,  we  do  not  consider  the  process  complete  until  we 
have  “checked”  the  timing.  The  simplest  way  to  check  the 
exhaust  valve  timing  is  to  crank  the  motor  in  direction  of  rota- 
tion until  the  exhaust  valve  is  just  closing.  Find  this  point 
accurately  as  I have  just  explained,  then  place  the  rod  through 
the  spark  plug  hole  and  make  a mark.  Now,  this  mark  repre- 
sents the  place  where  the  valve  actually  does  close.  Then,  if 
this  is  supposed  to  be  after  top  center,  let  us  back  the  engine 
until  we  reach  top  center.  Then,  make  another  mark.  By 
measuring  the  distance  between  these  marks,  we  know 
exactly  where  the  exhaust  valve  is  closing. 

18.  The  same  method  of  checking  applies  to  spark  timing  with 
the  exception  that  we  watch  the  breaker  points  instead  of 
the  exhaust  valve  clearances. 


34 

19.  Occasionally  a bad  cam  shaft  is  received  from  the 
factory.  If  we  had  a mysterious  trouble  in  the  engine  that 
no  one  could  find  the  cause  of,  it  would  be  well  to  check  the 
valve  timing  in  every  cylinder  of  the  motor,  and  check  not 
only  the  closing  of  the  exhaust  valves,  but  evety  operation 
which  the  cam  shaft  performs. 


LECTURE  IV. 

FUEL  AND  CARBURETION. 

1.  Nearly  all  the  fuels  now  in  use  for  internal  combustion 
motors  are  distilled  from  petroleum.  Alcohol  is  not  used  to 
any  extent  as  yet.  I will  explain  briefly  the  methods  of  dis- 
tilling lighter  fuels  from  petroleum.  The  petroleum  is  heated 
usually  by  means  of  steam  pipes  in  order  to  avoid  danger  of 
fire,  and  when  heated  to  a comparatively  low  temperature,  at 
first  the  lighter  elements  come  off'  in  the  form  of  vapor . The  first 
vapor  to  come  off,  when  condensed,  will  be  the  liquid  known 
as  ether.  If  the  petroleum  is  heated  to  a higher  temperature, 
high  test  gasoline  will  come  off  in  the  form  of  vapor.  Then, 
by  heating  the  petroleum  to  a greater  temperature,  low  test 
gasoline  will  come  off.  Now  as  to  the  next  products,  that  will 
depend  upon  the  kind  of  crude  oil  used. 

2.  There  are  two  general  classes  of  crude  oil  or  petroleum. 
Most  of  the  eastern  oil  is  what  is  known  as  paraffine  base  oil. 
Most  of  the  western  is  known  as  asphalt  base  oil.  In  the  case 
of  the  paraffine  base,  after  the  low  test  gas,  next  comes  kero- 
sene, usually  in  large  quantities.  In  the  case  of  the  asphalt 
base,  the  next  product  below  gasoline  is  distillate  or  naphtha. 
After  kerosene  or  distillate  we  get  light  lubricating  oils,  and 
by  heating  the  crude  oil  to  a still  higher  temperature  we  obtain 
cylinder  oils  or  heavier  lubricating  oils  and  so  on  through  the 
oils  until  we  come  to  the  tar  products  and  vaseline.  The 
chemists  divide  these  various  products  into  many  different 
varieties. 

3.  I will  explain  what  is  meant  by  high  test  and  low  test 
gasoline.  High  test  gasoline  is  very  light  and  volatile  and 
evaporates  very  rapidly  if  left  open,  while  lowT  test  is  heavier, 
less  volatile  and  will  not  evaporate  so  easihr  The  gasolines 
are  tested  by  the  Baume  gravity  scale.  Formerly  the  gasoline 
of  high  test  would  show  72  degrees  on  test,  but  at  present,  ordi- 
nary automobile  gasoline  tests  about  60  de  rees. 


35 

4.  As  the  demands  for  gasoline  have  increased  in  the  last 
few  years,  the  oil  refineries  have  been  forced  to  mix  many  of 
the  heavier  fuels  with  gasoline.  For  instance,  distillate,  and 
then  in  order  to  make  the  gas  test  properly,  they  have  added 
some  of  the  higher  test  fuels.  In  other  words,  the  present  day 
gasoline  may  be  a mixture  of  everything  above  gasoline  and 
a good  deal  that  is  below,  and  mixed  in  such  proportions  that 
the  result  will  serve  as  gasoline. 

5.  There  is  also  another  method  of  obtaining  gasoline. 
This  is  by  compressing  natural  gas  to  a very  high  pressure  and 
practically  wringing  liquid  gasoline  from  it.  For  instance, 
after  oil  wells  have  been  pumped  dry,  they  often  yield  gas 
which  is  useful  for  this  purpose  and  in  certain  districts  there 
are  gas  wells  which  are  useful  in  the  same  way.  This  gasoline 
is  known  as  “casing-head”  gasoline. 

6.  Carburetion  is  the  process  of  mixing  fuel  with  the  air. 
Gasoline  will  burn  rapidly  and  cleanly  only  when  mixed  with  a 
large  proportion  of  air.  Now  the  problem  is  to  mix  the  gaso- 
line and  air  rapidly  in  a comparatively  small  and  light  mixing 
device.  That  is  the  first  part  of  the  problem,  we  will  say. 
Originally,  with  the  first  gasoline  engines,  the  gasoline  used 
was  of  high  test  and  would  combine  or  mix  with  the  air  very 
easily.  In  fact  it  was  only  necessary  to  draw  air  over  a large 
surface  wetted  with  gasoline  and  the  air  would  become  satu- 
rated with  the  gasoline  vapor,  and  was  readily  burned  in  the 
motor.  But  the  present  day  gasoline  will  not  mix  in  this 
manner. 

7.  There  are  two  methods  for  mixing  gasoline  with  air. 
The  first  is  to  break  the  gas  into  very  fine  spray,  and  the  other 
is  to  vaporize  it  by  heat.  Usually  a combination  of  these  two 
methods  is  used. 

8.  I will  now  try  to  make  plain  the  method  of  breaking  the 
gasoline  into  a fine  spray.  Let  us  start  with  the  stationary 
gasoline  engine,  running  at  a constant  speed.  We  will  have  a 
straight  pipe,  for  instance,  1J  inch  in  diameter,  running  to  the 
gas  engine  cylinder  so  that  the  piston  will  draw  air  through 
this  pipe  during  the  suction  stroke.  Then  we  will  have  a 
nozzle  or  jet  in  this  pipe,  placed  in  Such  a way  that  all  air 
must  pass  it.  We  will  feed  gasoline  to  this  nozzle  from  a float 
chamber  located  beside  and  just  outside  the  pipe.  This  float 
chamber  will  be  so  arranged  that  when  the  gasoline  reaches  a 
certain  height,  for  instance,  even  with  the  top  of  the  nozzle,  the 
float  will  close  the  valve  and  stop  the  gasoline  from  rising  any 
higher.  This  is  a simple  device  for  the  purpose  of  maintaining 
the  gasoline  at  a constant  level.  With  an  arrangement  like 


36 

this,  the  gas  will  feed  through  the  nozzle  equally  well  with  the 
tank  full  or  nearly  empty.  Now  as  the  air  goes  through  this 
pipe  to  the  cylinder,  it  will  pass  the  spray  nozzle  with  consid- 
erable speed  and  will  pick  up  or  suck  up  a certain  amount  of 
gasoline  from  the  nozzle.  In  this  way  the  gas  will  come  out 
of  the  nozzle  in  a fine  spray  and  will  be  fairly  well  mixed  with 
the  air  by  the  time  it  reaches  the  cylinder,  and  during  the  com- 
pression stroke  the  heat  of  the  cylinder  and  the  heat  of  com- 
pression will  help  unite  the  particles  of  gasoline  with  the  air. 

9.  By  using  a simple  carburetor,  such  as  I have  described, 
if  we  should  try  to  slow  the  motor  down  by  closing  the  throttle 
valve  between  the  spray  nozzle  and  the  cylinder,  the  air  wrould 
flow  past  the  nozzle  so  slowly  that  it  would  no  longer  pick  up 
gasoline,  or  in  any  event,  it  would  not  flow  rapidly  enough  to 
break  up  the  gas,  even  if  it  should  draw  some  out  of  the  noz- 
zle. So  we  see  that  our  carburetor  is  not  suitable  for  vari- 
able speed. 

10.  Our  next  step  will  be  to  put  a choke  tube  in  this  carbu- 
retor, or  in  other  words,  a restricted  air  passage  at  the  point 
where  the  spray  nozzle  is  located.  By  this  means  we  can  make 
the  air  passage  so  small  that  even  when  the  engine  is  running 
quite  slowly  there  will  be  sufficient  air  velocity  at  the  spray 
nozzle  to  pick  up  and  break  up  the  fuel. 

11.  Having  decided  to  restrict  this  air  passage  at  this  point, 
the  next  problem  is  of  what  form  shall  we  make  this  choke 
tube.  We  must  choose  a form  of  choke  tube  which,  wffiile  it 
is  small  enough  to  get  the  desired  air  velocity  at  low  speed, 
will  still  permit  a large  amount  of  air  to  flow  through.  We 
find  that  the  Venturi  form  of  air  passage  is  best  adapted  to 
this  purpose.  It  is  practically  an  air  nozzle  so  formed  that 
the  side  in  which  the  air  enters  is  at  rather  an  abrupt  angle, 
but  the  side  through  which  the  air  leaves  is  at  a gradual  angle, 
or,  in  other  words,  allows  the  air  to  expand  gradually  on  the 
other  side.  This  has  the  following  advantages:  first,  that 
this  form  of  opening  allows  more  air  to  pass  through  for  a 
given  size  than  other  forms,  and  also  that  as  the  air  is  expand- 
ing as  it  passes  through,  there  is  a tendency  for  the  jet  of  fuel 
from  the  nozzle  to  burst  apart  into  a fine  spray. 

12.  Now,  having  put  this  choke  tube,  or  throat,  as  it  is 
sometimes  called,  in  our  carburetor,  we  will  see  how  it  works. 
Evidently  our  motor  will  run  nicely  at  slow  speed,  but  wffien 
we  open  the  throttle  to  allow  the  motor  to  run  very  fast,  we 
will  have  trouble  becasue,  as  the  suction  of  the  engine  increases, 
we  find  that  the  flow  of  gas  from  the  spray  nozzle  will  increase 
faster  than  the  flow  of  air  through  the  choke  tube.  This  is  in 


37 

accordance  with  a certain  law  of  physics.  This  means  that 
our  mixture  of  gasoline  and  air  will  become  richer  as  the 
engine  speeds  up;  richer  means  that  there  will  be  too  large  a 
proportion  of  gasoline  for  the  amount  of  air.  Therefore,  we 
must  find  some  means  of  diluting  this  mixture  at  high  speeds. 
We  will  make  a large  air  opening  between  the  spray  nozzle  and 
the  cylinder  and  we  will  put  a valve  in  this  opening  which  will 
be  held  closed  by  a light  spring.  This  spring  will  hold  the 
valve  closed  while  we  are  running  slowly,  but  as  the  motor 
runs  faster  and  the  mixture  tends  to  become  richer,  the  suc- 
tion will  open  the  valve  and  allow  enough  air  to  come  in  to 
dilute  the  mixture  and  make  it  right,  provided  the  spring  is 
properly  adjusted.  This  we  will  call  an  auxiliary  air  passage 
and  auxiliary  air  valve.  The  air  passage  in  which  the  nozzle 
is  located  should  be  called  the  main  air  passage. 

13.  This  diluting  of  the  mixture  at  high  speeds  is  one  of 
the  problems  of  carburetor  work.  One  form  of  carburetor 
accomplishes  this  by  having  two  jets,  one  working  on  the 
principle  which  I have  described  so  that  it  delivers  a richer 
and  richer  mixture  as  the  speed  is  increased,  and  the  other 
arranged  on  a different  principle  so  that  it  delivers  a rarer  and 
rarer  mixture;  in  this  way  the  two  jets  compensate  each  other. 
We  will  take  up  this  particular  carburetor  later. 

14.  We  find  many  forms  of  carburetors,  but  they  are  nearly 
all  made  on  these  principles  that  I have  explained. 

15.  The  action  of  the  spring  on  the  auxiliary  air  valve  is 
considered  faulty  because  when  we  suddenly  open  the  throttle, 
there  is  a tendency  for  the  valve  to  jump  suddenly  and  move 
too  far  off  its  seat.  The  result  is  that  it  temporarily  admits  too 
much  air,  “ starving’ ’ the  motor.  To  correct  this,  some  manu- 
facturers use  what  is  known  as  a dash  pot  to  steady  the  action 
of  the  auxiliary  air  valve.  This  is  simply  a piston  working  in 
gasoline,  or  sometimes  in  air,  in  much  the  same  manner  as  the 
piston  in  a shock-absorber.  As  the  air  or  gasoline  is  forced  to 
pass  through  a comparatively  small  hole  in  the  piston,  the 
piston  cannot  move  rapidly.  Some  designers  of  carburetors 
go  still  further  and  in  addition  to  using  the  dash  pot  to  pre- 
vent the  air  valve  from  opening  too  far  or  too  suddenly,  they 
use  what  is  called  a metering  pin,  which  is  an  arrangement 
whereby  the  opening  of  the  auxiliary  air  valve  will  tempora- 
rily feed  an  extra  supply  of  gasoline  to  avoid  the  tendency  of 
starving  the  motor  when  the  throttle  is  suddenly  opened. 

16.  Some  carburetors  have  fixed  jets  of  certain  wire-gauge 
sizes  which  are  screwed  into  the  carburetor  and  cannot  be 
adjusted.  Others  have  nozzles  with  a pin  point  valve  screwed 


38 


in  or  out  of  them  to  change  the  size  of  the  opening  and  the 
amount  of  gasoline  flowing  through  them.  These  are  called 
needle  valves.  Some  carburetors  have  the  needle  valve  con- 
nected to  the  throttle  by  mechanical  means,  in  such  way  that 
the  needle  valve  will  be  opened  slightly  as  the  throttle  is 
opened,  or,  in  other  words,  as  the  speed  of  the  motor  is 
increased. 


Carburetor  Adjustment. 

17.  Let  it  be  clearly  understood  that  when  a mixture  con- 
tains too  large  a proportion  of  gasoline,  it  is  called  rich  and  the 
motor  is  said  to  be  flooded,  and  when  it  contains  too  small  a 
proportion  of  gasoline,  the  mixture  is  rare,  and  the  motor  is 
said  to  be  starved. 

18.  When  a motor  is  cold  it  usually  requires  a rich  mixture 
to  make  it  start  easily.  This  is  because  the  gasoline  does  not 
combine  readily  with  the  air.  In  order  to  accomplish  this 
temporary  richness  of  mixture  for  starting,  most  carbu- 
retors have  a device,  usually  called  a priming  or  flushing  pin, 
which  will,  when  operated,  depress  the  float  or  hold  the  float 
down  in  the  float  chamber  of  the  carburetor,  allowing  the  gaso- 
line to  rise  so  high  that  it  will  overflow  through  the  spray 
nozzle  of  the  carburetor,  thus  wetting  the  inside  of  the  carbu- 
retor with  gasoline  and  making  it  easy  for  the  air  passing 
through  to  pick  up  a heavy  charge  of  fuel. 

19.  Another  method  is  to  have  some  form  of  valve  between 
the  spray  nozzle  and  the  place  where  the  air  enters  the  carbu- 
retor. By  closing  this  valve  we  get  greatly  increased  suction 
in  the  spray  nozzle,  thus  picking  up  an  excess  of  gasoline. 
Such  a device  is  usually  called  a choker.  It  is  usually  oper- 
ated from  the  pilot’s  seat  while  the  motor  is  being  cranked. 

20.  Most  carburetors  have  two  gasoline  adjustments  and 
an  air  adjustment;  one  gasoline  adjustment  for  low  speed  and 
one  for  high  speed.  In  addition  to  this,  we  have  a little  set 
screw  which  allows  the  throttle  valve  to  close  more  or  less 
completely,  according  to  its  adjustment.  This  is  to  permit 
the  motor  to  idle  at  the  desired  speed. 

21.  Before  adjusting  any  carburetor,  the  motor  must  be 
thoroughly  warmed  up  and  ignition  system  must  be  perfect. 
The  valve  operation  and  compression  must  be  perfect,  and 
fuel  must  flow  in  a generous  stream  to  the  carburetor.  Also 
the  carburetor  float  chamber  and  spray  nozzle  must  be  clear. 
If  there  are  any  loose  joints  in  the  pipe  between  the  carburetor 
and  the  cylinders,  too  much  air  will  leak  in  and  dilute  the 
mixture.  Therefore,  these  joints  must  be  tight. 


39 


22.  I will  try  to  explain  in  a general  way  how  carburetors 
are  adjusted.  First,  all  adjustments  should  be  in  a medium 
position.  In  other  words,  half-way  between  the  most  and 
the  least,  of  either  air  or  gas  according  to  their  functions. 
The  low  speed  gasoline  adjustment  should  be  opened  two  or 
three  turns.  The  engine  should  be  started  and  thoroughly 
warmed  up.  Then  we  will  adjust  the  low  speed  gasoline 
adjustment  and  after  we  have  adjusted  it  so  that  we  are 
getting  the  best  results,  we  can  adjust  the  high  speed  gasoline 
adjustment  while  the  motor  is  running  at  high  speed.  Then 
having  our  mixture  right,  we  can  adjust  the  screw  which  limits 
the  closing  of  the  throttle  until  the  motor  runs  at  the  desired 
speed  for  slow  speed.  If  we  try  to  make  the  motor  run  too 
slow,  it  will  stop,  and  if  we  allow  it  to  run  too  fast  it  will  make 
landing  of  the  airplane  difficult  and  there  will  be  danger  of 
the  machine  running  into  and  injuring  the  man  who  cranks 
the  propeller  to  start  the  motor. 

Now  we  have  made  our  gasoline  adjustment  suit  the  adjust- 
ment of  the  auxiliary  air  valve.  If  the  auxiliary  air  valve  is 
unnecessarily  tight  our  motor  will  throttle  down  nicely  and 
respond  nicely  when  opened  quickly  but  will  fail  to  show  the 
proper  speed.  Then,  in  that  case,  by  loosening  the  auxiliary 
air  valve  spring,  or  allowing  the  auxiliary  air  valve  to  open 
further,  we  will  get  a larger  volume  of  air  through  the  carbu- 
retor, giving  the  motor  more  complete  charges  of  gas,  and 
gain  higher  speed.  Accordingly,  the  gasoline  adjustments  will 
have  to  be  reversed  to  suit  this  change  in  the  air  adjustment. 
In  most  of  these  carburetors  we  can  make  our  gasoline  adjust- 
ments suit  almost  any  air  adjustment;  and  in  airplane  work, 
the  amount  of  speed  or  revolutions  per  minute  we  can  make  is 
the  most  important  part  of  our  adjustment;  therefore,  we 
generally  aim  to  have  the  air  valve  open  as  far  as  possible. 
The  limit  to  this  is  that  if  the  air  valve  opens  too  far  our  spring 
is  too  weak,  and  the  motor  will  take  air  through  this  valve 
instead  of  through  the  main  air  passage  when  we  throttle 
the  motor  down,  and  also  it  will  fail  to  respond  when  we 
open  the  throttle  quickly.  So  let  us  see  that  we  have  the 
auxiliary  air  valve  adjusted  so  that  it  will  just  stay  seated  or 
closed  when  the  motor  is  running  very  slowly  or  idle,  but  not 
tighter  than  is  necessary. 

23.  Most  carburetors  are  provided  with  a means  of  drawing 
hot  air  from  the  outside  of  the  exhaust  pipe,  for  the  heat  helps 
to  evaporate  the  particles  of  gasoline,  or  in  other  words,  helps 
the  mixing  of  the  gas  with  the  air. 


40 

24.  If  cold  air  only  is  used,  or  if  the  motor  itself  is  cold,  the 
gasoline  goes  into  the  cylinders  in  “chunks,”  and  therefore 
it  is  never  completely  mixed  with  the  air  and  is  never  completely 
burned.  The  result  is  that  a larger  amount  of  gasoline  must 
be  fed  to  the  motor.  When  heat  is  used,  or  when  the  motor  is 
hot,  the  heat  causes  the  gasoline  to  combine  more  thoroughly 
with  the  air  and  less  fuel  can  be  used.  The  result  is  that  if 
we  should  adjust  the  carburetor  properly  when  the  motor  is 
cold,  the  mixture  would  become  too  rich  when  the  motor  was 
thoroughly  warmed  up,  which  would  make  poor  economy  and 
cause  trouble.  If  we  adjust  the  carburetor  properly  while  the 
motor  is  hot,  then  the  motor  may  be  difficult  to  start  when  it  is 
cold  and  may  run  badly  until  it  is  well  warmed  up,  but  this  can 
be  taken  care  of  by  using  the  choker,  which  I have  described 
before.  The  main  thing  is  to  have  a correct,  economical  and 
clean  mixture  after  the  motor  is  heated  up. 

25.  If  the  mixture  is  too  rich,  it  burns  very  slowly.  The 
result  is  that  the  charge  will  be  completely  burned  and  the 
maximum  temperature  reached  only  after  the  piston  has  de- 
scended in  the  cylinder  for  a considerable  distance.  Thus,  the 
greatest  heat  occurs  at  a time  when  there  is  a large  amount  of 
surface  exposed  to  absorb  it.  The  result  is  over-heating  of 
the  motor;  also  the  heat  will  be  very  great  at  the  time  the 
exhaust  valve  opens,  which  would  burn  the  exhaust  valve  and 
overheat  the  exhaust  pipe.  We  lose  power  also,  because  the 
maximum  pressure  occurs  late,  after  the  piston  has  already 
completed  part  of  the  working  stroke. 

If  the  mixture  is  too  rare,  first  of  all,  we  lose  power  and  have 
excessive  vibration.  Next,  if  it  is  still  rarer,  the  motor  will 
“spit”  through  the  carburetor.  This  is  known  as  a back-fire. 
It  is  supposed  to  be  caused  in  this  manner:  This  rare  mixture 
also  burns  very  slowly,  so  slowly  in  fact  that  there  will  be  a 
flame  remaining  in  the  cylinder  even  after  the  exhaust  stroke, 
and  when  the  inlet  valve  opens  to  admit  a new  charge,  the 
flame  remaining  in  the  cylinder  will  ignite  the  gas  and  the  flame 
will  travel  down  the  inlet  pipe  to  the  carburetor.  This  is 
dangerous  because  it  is  likely  to  set  fire  to  the  airplane,  espe- 
cially if  the  gasoline  pipes  are  leaking  slightly,  near  the  car- 
buretor, which  they  often  are. 

The  Zenith  Carburetor. 

26.  This  carburetor  is  used  all  over  the  world  on  airplane 
engines.  It  is  of  the  so-called  non-ad justable  type.  The  only 
method  of  adjusting  the  carburetor  is  to  remove  the  jets  or 


41 

the  choke  tube  and  replace  them  with  other  sizes.  These 
carburetors  are  fitted  with  the  proper  size  jets  and  chokes  at  the 
factory  where  the  motors  are  tested,  and  never  require  further 
adjustment.  The  Zenith  is  of  the  type  of  carburetor  having 
no  auxiliary  air  valve  but  using  the  compensating  jet  prin- 
ciple. In  other  w6rds,  it  has  a main  jet  which  delivers  a 
richer  and  richer  mixture  as  the  suction  is  increased  and  a com- 
pensating jet  which  is  arranged  to  deliver  a rarer  and  rarer 
mixture  as  the  suction  is  increased.  To  make  the  operation 
of  these  two  jets  plain,  we  will  suppose  that  I am  sucking 
lemonade  out  of  a bottle,  which  corresponds  to  the  “float 
chamber,”  with  a straw  corresponding  to  the  main  jet.  The 
harder  I suck  on  the  straw,  the  more  lemonade  I can  get 
(richer  and  richer  mixture).  Now,  for  the  compensating  jet. 
Suppose  you  pour  lemonade  from  a bottle,  drop  by  drop 
into  an  open  glass,  this  glass  corresponding  to  the  “well”  in 
the  carburetor.  I would  suck  from  the  glass  or  “well”  through 
a straw  (corresponding  to  the  priming  tube  or  cap  jet).  No 
matter  how  hard  I suck,  I get  only  the  amount  that  you  drop 
into  the  glass,  and  some  air.  As  1 increase  the  suction  in  the 
straw,  and  cannot  get  any  more  gas,  I get  more  and  more  air, 
or,  the  mixture  I get  will  be  thinner,  rarer  and  rarer. 

27.  Now,  I will  try  to  explain  the  operation  of  the  carbu- 
retor. (The  names  of  the  parts  on  the  carburetor  should  be 
taught  on  the  model  at  this  point.)  As  we  come  to  start* the 
motor,  we  find  that  the  gasoline  has  gradually  flowed  through 
the  compensating  jet  and  has  filled  the  “well”  to  a point  even 
with  the  top  of  the  main  jet.  If  the  motor  is  cold,  we  must 
close  the  throttle  to  take  the  charge  and  as  the  pistons  can 
draw  air  only  through  the  priming  tube,  they  will  suck  up 
liquid  in  the  well  through  the  priming  tube  until  the  well  is 
empty,  thus  giving  us  a rich  mixture  for  starting  the  cold 
motor. 

28.  Now  if  we  open  the  switch  and  start  the  motor,  it  will 
run  slowly  with  the  throttle  closed  or  nearly  closed,  and  it 
will  still  be  sucking  only  through  the  priming  tube.  But  as 
the  compensating  jet  supplies  gasoline  to  the  well  and  priming 
tube  in  very  small  quantities,  the  pistons  will  draw  far  more 
air  than  gasoline  through  the  priming  tubes.  Therefore  we 
can  think  of  this  priming  tube  as  a miniature  carburetor  for 
running  the  motor  idle.  Now,  if  we  open  the  throttle  slightly, 
the  suction  will  be  decreased  in  the  priming  tube  and  increased 
in  the  choke  tube  of  the  carburetor  in  the  main  air  passage. 
Therefore  the  suction  will  naturally  be  transferred  to  the  cap- 
jet,  or  outer  jet,  placed  in  the  choke  tube.  We  are  now  draw- 


42 

ing  this  same  amount  of  gasoline  and  some  air  from  the  “well’' 
and  compensating  jet,  but  through  a new  route,  viz.,  the  cap- 
jet.  Then  as  we  open  the  throttle  a little  more,  the  air 
velocity  through  the  choke  tube  will  increase  and  the  main  jet 
begin  to  deliver  a small  amount  of  gasoline.  This  main  jet, 
you  will  remember,  is  the  type  which  will  deliver  more  and 
more  gas  as  the  suction  is  increased.  We  can  call  this  main 
jet  our  high-speed  jet. 

29.  As  we  use  the  priming  tube  for  slow  running,  it  is  not 
necessary  to  have  the  choke  tube  very  small,  therefore  the 
suction  \till  not  increase  around  the  main  jet  to  the  extent 
that  it  does  in  most  carburetors.  Also,  the  main  jet  is  not 
quite  large  enough  to  supply  the  gas  needed  at  high  speed  and 
relies  partly  on  the  jet  which  is  fed  from  the  compensating 
jet,  and  as  this  latter  jet  delivers  a rarer  and  rarer  mixture,  it 
will  offset  any  tendency  on  the  part  of  the  main  jet  to  deliver 
a richer  and  richer  mixture;  thus,  we  will  say,  the  jets  compen- 
sate each  other. 

30.  This  carburetor  is  provided  with  the  usual  screw  adjust- 
ment to  limit  the  closing  of  the  throttle  valve  in  order  to  make 
the  motor  idle  at  the  proper  speed,  and  also  there  is  a small 
air  adjustment  which  acts  on  the  priming  tube  and  affects 
the  running  of  the  motor  only  at  the  idling  speed. 

31.  To  take  a charge  when  the  motor  is  cold,  the  throttle 
should  be  completely  closed,  and  when  the  motor  is  not  cold, 
it  should  be  very  slightly  opened,  because,  if  we  took  a charge 
with  the  throttle  closed,  while  the  motor  was  hot,  there  would 
be  danger  of  getting  entirely  too  much  gasoline  in  the  cylin- 
ders, or  flooding  the  motor.  That  is,  there  would  be  such 
a large  proportion  of  gasoline  to  air  that  it  would  be  impos- 
sible to  ignite  the  mixture. 

Action  of  Gasoline  in  the  Inlet  Pipes  and  Manifold. 

32.  After  adjusting  our  carburetor  to  deliver  the  proper 
mixture  of  gasoline  and  air  under  all  conditions,  our  problem 
is  not  yet  completely  solved.  We  have  still  to  solve  the  prob- 
lem of  keeping  the  gas  mixed.  As  the  mixture  leaves  the 
carburetor,  it  consists  of  air  and  a finely  divided  spray  of 
gasoline.  If  we  could  see  this,  it  would  resemble  steam  from  a 
tea-kettle.  Now,  this  mixture  must  be  kept  rapidly  moving 
or  it  will  condense,  that  is,  the  liquid  will  gather  together  in 
puddles  in  all  the  low  places  on  the  way  to  the  cylinders. 

33.  One  method  of  avoiding  this  condensation  is  by  allowing 
hot  air  to  flow  to  the  carburetor  or  by  heating  the  intake 


43 

manifold  from  the  outside  so  that  the  walls  will  be  sufficiently 
hot  to  vaporize  the  liquid  fuel  which  comes  in  contact  with 
these  walls.  It  is  important  to  have  the  manifold  of  the  same 
inside  diameter-  as  the  out-let  of  the  carburetor  because  if  it 
is  larger  the  mixture  will  flow  more  slowly  than  it  would  in  the 
carburetor  and  condensation  will  be  the  result.  Allowing  hot 
air  to  flow  through  the  carburetor  helps  the  gasoline  spray  to 
evaporate  or  combine  with  the  air,  but  we  must  remember 
that  if  the  mixture  is  heated  too  much,  it  goes  into  the  cylinder 
in  a hot  and  expanded  condition,  and,  therefore,  we  don’t  get 
as  much  of  it  in  the  cylinder  to  compress.  In  other  words,  we 
do  not  get  a good,  full  charge,  and  there  is  less  oxygen  to  the 
cubic  foot  of  the  heated  mixture. 

34.  Another  problem  connected  with  the  handling  of  the 
mixture  of  gasoline  and  air  is  that  with  certain  motors  there  is 
a tendency  for  the  gas  to  reciprocate  in  the  manifold,  or,  in 
other  words,  to  jump  back  and  forth.  This  is  especially  true 
in  a six-cylinder  vertical  motor  with  the  usual  firing  order. 
The  gas  will  be  drawn  towards  one  end  of  the  engine  by  one 
cylinder  and  next  it  will  be  drawn  in  the  opposite  direction 
by  a cylinder  in  the  other  end  of  the  motor ; thus  a very  hard 
condition  has  to  be  overcome  and  causes  uneven  gas  distribu- 
tion and  condensation.  The  same  condition  exists  in  eight- 
cylinder  “ V ” type  motors  if  a single  carburetor  is  used,  because 
there  is  a variation  in  the  suction  from  the  two  sides  of  the 
motor  and  a tendency  for  the  gas  to  reciprocate  between  the 
two  sides. 

35.  Most  of  the  modern  airplane  engines  of  six-cylinder 
vertical  and  eight  and  twelve  cylinder  “ V ” type  motors  use  a 
divided  inlet  manifold  and  a duplex  or  double-barrel  carbu- 
retor. This  effectually  prevents  any  possibility  of  the  gas 
reciprocating  from  one  half  of  the  motor  to  the  other  because 
they  get  their  gas  from  separate  sources.  The  “ duplex” 
carburetors  have  a single  float  chamber  to  supply  the  two  sets 
of  jets. 


THINGS  TO  BE  REMEMBERED  ABOUT 
CARBURETORS. 

36.  It  is  impossible  to  get  a correct  adjustment  on  a car- 
buretor unless  everything  else  about  the  motor  is  right.  If  a 
carburetor  seems  to  need  very  exact  adjustment,  or,  in  other 
words,  the  motor  seems  to  be  sensitive,  it  is  a pretty  sure 
sign  that  something  is  not  quite  right.  For  instance,  the 
ignition  may  be  poor,  or  there  may  be  air  leaks  in  the  inlet 


44 


manifold,  or  dirt  in  the  carburetor,  and  under  these  circum- 
stances, no  one  can  get  a good  adjustment.  If  a carburetor 
d ips  gas,  that  is,  seems  to  leak,  it  may  be  that  the  float  is  set 
too  high  and  shuts  off  the  gasoline  only  after  it  is  high  enough 
to  overflow  through  the  spray  nozzle.  Or  it  may  be  that  there 
is  dirt  under  the  float  valve  which  keeps  it  from  seating 
properly.  It  may  be  a defective  or  leaky  float  valve.  Fre- 
quently, when  the  carburetor  drips  gasoline  there  is  nothing 
the  matter,  and  the  gasoline  dripping  out  is  simply  due  to  the 
fact  that  after  we  stop  the  motor  the  gas  which  was  in  the 
manifold  or  inlet  pipe  will  condense  and  drip  out  through  the 
carburetor.  Therefore,  when  our  carburetor  leaks,  we  must 
notice  whether  or  not  it  occurs  as  soon  as  we  turn  on  the  gaso- 
line before  starting  the  motor,  in  which  case  it  would  be 
trouble  with  the  float;  or  whether  it  only  leaks  after  running 
the  engine,  in  which  case  it  would  be  due  to  condensation  and 
unavoidable. 

37.  Back-firing  in  a carburetor  may  be  caused  by  too  rare 
mixture  or  by  an  extremely  rich  one.  Usually,  however,  it  is 
a rare  mixture.  Either  mixture  can  burn  so  slowly  that  there 
will  be  enough  flame  left  in  the  cylinder,  even  after  the  scaveng- 
ing stroke,  to  ignite  the  new  incoming  charge  of  gas  when  the 
inlet  valve  opens.  Sometimes  the  flame  will  reach  the  car- 
buretor or  make  a noise.  A leaky  inlet  valve  can  also  cause 
a back-fire  if  it  leaks  badly  enough  or  sticks  open.  An  engine 
can  “pop”  from  two  causes,  either  from  back-fire  in  the 
carburetor,  which  would  indicate  lack  of  gas  or  too  much  air 
or  possibly  inlet  valve  trouble.  The  other  cause  for  “pop- 
ping” can  be  a very  rich  mixture  or  a very  late  spark.  But 
there  is  a great  difference  in  these  two  because  in  the  case  of 
the  former  the  pop  will  occur  in  the  carburetor  and  in  the  case 
of  the  latter,  it  will  be  in  the  exhaust  ports  or  exhaust  pipes. 
So  it  is  necessary  to  determine  where  the  pop  is  occurring  be- 
fore attempting  lo  find  the  cause. 

COLD  WEATHER  STARTING. 

38.  In  cojd  weather  many  people  prime  a motor  with  gaso- 
line direct  in  the  cylinder  to  help  start  it.  There  are  objec- 
tions to  this  method.  For  instance,  if  the  motor  fails  to  start 
on  the  first  priming,  we  are  likely  to  find  that  we  are  losing 
compression  because  the  gasoline  has  washed  the  oil  off  the 
piston  rings,  and  when  the  motor  does  finally  start  there  will 
be  danger  of  scoring  or  scratching  the  cylinder  walls,  on  account 
of  the  oil  not  being  there  to  lubricate. 


45 

39.  A better  method  of  priming  the  motor  is  to  turn  it  over 
as  fast  as  you  can  with  the  switch  safe  or  closed,  and  the 
throttle  half  open,  and  while  this  is  being  done,  completely 
stop  the  auxiliary  air  opening  with  one  hand  and  nearly  stop 
the  main  air  passage  with  the  other.  This  will  cause  high  air 
velocity  and  suction  at  the  spray  nozzle.  This  method  gets 
the  gas  into  the  cylinder  in  the  form  of  a vapor  or  spray.  The 
result  is,  the  motor  is  more  apt  to  start  and  there  is  less  danger 
of  scoring  the  cylinders.  Frequently  it  is  possible  to  know 
when  you  have  a charge  in  the  motor  because  you  can  see  the 
vapor  coming  out  of  the  exhaust  pipes  by  looking  very  closely. 
This  applies  to  the  case  where  the  motor  is  cold.  If  the  motor 
is  hot  there  would  be  a certain  amount  of  vapor  present  from 
other  causes.  Another  good  method  of  priming  is  to  squirt 
gas  through  a cock  in  the  top  of  the  intake  manifold  or  at  the 
highest  point  of  the  manifold.  In  this  way,  the  gasoline  will 
spread  over  the  interior  walls  of  the  pipe  and  expose  a large 
wetted  surface  for  the  air  to  pass  over.  In  extreme  cases,  it 
is  a help  to  saturate- a piece  of  rag  with  “ ether”  and  hold  over 
the  air  intakes  of  the  carburetor  while  taking  a charge.  The 
motor  can  be  started  in  this  way  when  it  refuses  to  start  in  any 
other  way. 

40.  Nine  times  out  of  ten  when  the  motor  seems  to  have 
carburetor  trouble  the  trouble  is  somewhere  else.  It  seems  as 
though  the  human  race  is  prone  to  blame  the  carburetor  for 
all  troubles,  possibly  because  it  is  the  easiest  thing  to  “ monkey” 
with.  Many  times  I have  seen  people  adjusting  a carburetor 
to  correct  a trouble  caused  by  the  magneto  being  wired  up 
wrong.  If  the  engine  is  missing  or  acting  badly  in  any  way 
and  we  believe  the  carburetor  is  to  blame,  we  can  find  out  or 
prove  it  in  the  following  manner.  If  we  have  a carburetor  of 
the  auxiliary  air  valve  type,  simply  run  the  engine  at  the  speed 
where  it  is  giving  trouble  and  raise  the  air  valve  slightly  off 
its  seat  with  one  finger,  thus  temporarily  making  the  mixture 
rarer;  and  if  this  does  not  correct  the  trouble  try  holding  the 
hand  partially  over  the  air  intake  to  make  the  mixture  tem- 
porarily richer.  If  neither  of  these  operations  stops  the  miss- 
ing or  trouble,  the  fault  absolutely  is  not  in  the  carburetor,  and 
if  raising  the  air  valve  or  thinning  the  mixture  would  seem  to 
help  the  engine,  that  might  only  mean  that  there  was  dirt 
under  the  float  valve  allowing  gasoline  to  rise  too  high  in  the 
float  chamber,  and  would  not  necessarily  mean  that  the  ad- 
justments of  the  carburetor  should  be  changed.  It  might  also 
mean  that  we  should  have  had  more  cold  air  and  less  heated 
air.  If  conditions  seem  to  be  improved  by  putting  our  hand 
over  the  air  intake  or  making  the  mixture  richer  that  might 


45 

mean  that  we  had  air  leaks  in  the  inlet  manifold,  no  air  vent 
in  the  gas  tank,  poor  flow  of  gasoline  to  the  carburetor  through 
feed  pipes,  water  in  the  gasoline,  dirt  in  the  spray  nozzle,  or 
weak  exhaust  valve  springs.  The  chances  are  that  there 
would  be  no  real  necessity  for  altering  the  carburetor  adjust- 
ments to  feed  more  gas. 

41.  Remember  that  we  must  not  leave  gasoline  in  the  float 
chamber  of  a carburetor  if  the  engine  is  not  going  to  be  run 
for  a week  or  so,  because  some  grades  of  gasoline  when  evapo- 
rating in  a carburetor  under  these  circumstances  will  leave  a 
deposit  which  resembles  wax  or  soft  soap,  and  naturally  when 
gasoline  is  turned  on  and  the  engine  started,  this  deposit  will  clog 
some  of  the  fine  gasoline  passages.  In  any  case  it  is  wise  to 
inspect  and  clean  the  carburetor  after  the  motor  has  been 
standing  for  some  time  without  running. 

42.  Make  it  a rule  never  to  connect  the  feed  pipe  to  the 
carburetor  without  first  allowing  gasoline  to  run  through  the 
pipe  to  clean  the  pipe  or  flush  it  out  and  to  prove  the  volume 
of  the  flow.  If  there  is  any  doubt  as  to  the  amount  of  the 
flow  through  the  pipes  being  sufficient  you  can  prove  it  in  this 
way:  Suppose  your  motor  burns  ten  gallons  of  gasoline  per 
hour.  You  should  not  be  satisfied  with  the  flow  of  gas  unless 
it  will  flow  twice  that  fast,  or  two  gallons  in  six  minutes. 
When  running  gas  through  the  pipe  for  the  purpose  of  cleaning 
hold  the  end  of  the  pipe  as  low  as  possible  to  insure  all  heavy 
dirt  and  water  coming  out  of  the  end  of  the  pipe,  and  when 
testing  the  amount  of  flow  try  to  have  the  end  of  the  pipe  at 
the  same  height  as  where  it  connects  with  the  carburetor  in 
order  to  make  it  a fair  test.  To  clean  out  the  carburetor,  we 
should  begin  at  the  gasoline  tank  and  see  that  there  is  a good 
air  vent  in  the  tank,  because  gasoline  cannot  flow  out  of  the 
tank  unless  air  is  able  to  come  in  and  take  its  place.  (Note. — 
This  refers  to  gravity  feed.) 

43.  Then  we  should  disassemble  and  clean  out  the  float 
chamber,  inspecting  the  float  mechanism  to  see  that  nothing 
is  working  loose.  We  should  clean  out  the  strainers,  wherever 
they  may  be  located;  clean  the  jets.  A special  socket  wrench 
should  be  used  to  remove  them  for  cleaning.  Always  remove 
one  at  a time,  clean  it  and  replace  it  firmly  before  removing 
another.  See  that  a gasket  is  on  it,  but  don’t  change  the 
thickness  of  the  gasket  as  this  would  affect  the  flow  from  the 
jet.  Sand,  water,  or  rust  flakes  are  likely  to  be  found  in  the 
jets.  In  the  Zenith  carburetor,  there  are  plugs  under  the  jets 
which  make  them  accessible.  These  plugs  also  have  a cup  in 
them  which  catches  dirt,  and  they  should  be  cleaned  out  before 
being  put  back  in  place. 


LECTURE  V. 

MAGNETOS. 

1.  In  speaking  of  electric  currents,  we  speak  of  the  flow  and 
the  pressure  in  much  the  same  way  in  which  we  would  speak 
of  the  amount  of  water  flowing  through  a pipe  or  of  the  pres- 
sure of  water  in  the  pipe.  In  electricity  the  amount  of  flow 
is  measured  in  amperage , or  number  of  amperes;  and  the  pressure 
is  called  voltage  or,  we  speak  of  the  number  of  volts  in  a circuit. 
Then  a high  pressure  current  would  be  called  a high  voltage 
current.  Low  pressure  would  be  called  low  voltage.  We 
also  use  the  expressions  high  tension  and  low  tension.  For 
ignition  in  a motor,  we  require  high  pressure  or  high  tension 
current,  which  will  be  able  to  force  its  way  through  the  space 
between  the  spark  plug  points.  A high  tension  magneto  does 
three  things:  it  generates  a current,  “steps  it  up”  and  dis- 
tributes it,  or  “hands  it  out”  to  the  proper  cylinder  at  the 
proper  time.  The  so-called  low  tension  magneto  generates 
a low  tension  current  only,  and  has  a simple  armature  com- 
posed of  a soft  iron  core  with  a single  winding  of  wire. 

2.  The  high  tension  magneto  has  a double-wound  arma- 
ture. It  is  constructed  with  a soft  iron  core,  then  a primary 
winding  composed  of  a few  turns  of  coarse  wire  next  to  the  coce. 
Outside  thus  comes  a secondary  or  outer  winding  composed 
of  very  fine  wire  and  having  perhaps  a thousand  times  as  many 
turns  as  the  primary  winding.  (Note. — The  names  of  the 
parts  on  a model  should  be  taught  at  this  point.)  As  the  ar- 
mature revolves  between  the  ends  of  the  magnets  or  in  the 
magnetic  field,  a low  tension  current  is  generated  in  the  pri- 
mary winding . W e call  this  the  prim  ary  circuit  (prim  ary  circuit 
not  useful  for  ignition),  and  by  suddenly  stopping  this  current, 
a strong  wave  of  high  tension  current  will  be  “induced”  in 
the  outer  or  secondary  winding.  This  high  tension  current 
is  collected  by  the  collector  ring  and  carried  by  the  collector 
brush  to  the  distributor  where  these  waves  of  current,  which 
we  call  “sparks,”  are  handed  out  to  the  right  cylinder  at  the 
right  time. 

3.  Now  in  order  suddenly  to  stop  or  break  this  primary 
current  it  must  flow  through  the  interruptor  or  breaker.  This 
device  makes  contact  for  a short  time  and  allows  the  current 
to  flow  through,  then  quickly  separates  the  contact  points, 
thus  breaking  the  circuit.  Now,  when  a current  is  flowing 

U7) 


48 

through  two  points,  it  possesses  a certain  amount  of  momen- 
tum or  speed  and  when  the  points  are  separated  it  tries  to 
jump  the  gap  and  keep  flowing  a fraction  of  a second  longer  in 
this  way.  But  if  it  does  continue  to  flow  we  will  not  have  made 
a quick  and  complete  break  in  the  primary  current  and  the 
spark  will  be  weak,  and  this  flowing  after  the  points  are  sep- 
arated (sparking)  will  burn  away  the  platinum  breaker  points > 
which  means  that  frequent  adjusting  and  filing  will  be  necessary 
to  keep  them  true.  This  tendency  of  the  current  to  keep  flowing 
after  the  break  occurs  is  taken  care  of  or  corrected  in  the  high 
tension  magneto  by  a device  called  the  condenser,  which  is 
capable  of  temporarily  absorbing  the  current,  which  would 
otherwise  try  to  jump  between  the  points  when  they  are  sepa- 
rated. When  the  condenser  of  a magneto  fails,  we  find  the 
breaker  points  burning  or  pitting,  and  we  find  the  spark  becom- 
ing weak.  An  electric  current  must  always  make  a complete 
circuit  or  round  trip,  returning  to  the  place  it  starts  from. 
I will  now  explain  the  routes  of  the  circuits  in  the  magneto. 
You  will  see  that  we  have  two  complete  and  separate  circuits 
in  a high  tension  magneto. 

The  Primary  Circuit. 

4.  The  primary  current  is  generated  in  the  inner  or  primary 
winding  armature  and  flows  from  one  end  of  this  winding  to 
the  breaker  and  condenser,  from  there  to  ground  or  frame  of 
the  engine  and  through  the  frame  of  the  magneto  to  the  core 
of  the  armature.  The  other  end  of  the  primary  winding 
is  connected  or  grounded  on  this  core,  so  in  that  way  the 
current  returns  to  the  place  where  it  started.  When  the 
breaker  points  separate,  the  current  no  longer  flowrs  through 
them  but  is  absorbed  by  the  condenser. 

The  Secondary  Circuit. 

5.  One  end  of  the  secondary  winding  is  grounded  to  the 
core  of  the  armature.  The  current  is  induced  in  this  winding 
at  the  time  of  the  break  in  the  primary  circuit  and  flows  to 
the  collector  ring,  and  through  the  collector  brush,  through  the 
connection  between  the  collector  brush  and  the  distributor  and 
from  the  distributor  to  the  central  or  insulated  electrodes  of 
the  spark  plug;  from  there  it  jumps  to- the  outer  or  grounded 
portion  of  the  spark  plug  through  the  frame  of  the  motor  and 
magneto  back  to  the  core  of  the  armature,  completing  the  cir- 
cuit. This  secondary  circuit  is  of  very  high  pressure,  some- 


49 

thing  like  30,000  volts,  and  must  therefore  be  very  heavily 
insulated.  If  one  of  the  wires  running  to  the  spark  plugs 
should  be  broken  and  the  ends  separated,  there  would  be  no 
outlet  for  this  particular  wave  of  secondary  current  and  it 
would  jump  to  the  ground  through  the  easiest  channel. 
Usually  it  would  jump  through  the  insulation  of  the  armature, 
puncturing  or  ruining  the  insulation.  To  avoid  this,  a safety 
valve  is  provided  which  is  called  the  “safety  spark  gap.” 
This  is  a gap  provided  on  the  magneto  which  has  the  points 
separated  a great  deal  farther  than  the  points  of  the  spark 
plug,  but  close  enough  so  that  the  resistance  will  be  less  than 
the  resistance  of  the  insulation  in  the  armature.  In  other 
words,  it  will  be  easier  for  the  spark  to  jump  the  safety  gap 
than  to  jump  through  the  insulation  in  the  armature,  but  it 
will  be  much  harder  to  jump  the  safety  gap  than  the  gap  in 
the  spark  plugs.  Then  if  we  find  a spark  occurring  in  this 
safety  gap,  that  should  tell  us  that  the  secondary  current 
finds  no  way  to  get  to  the  ground.  These  safety  gaps  are 
enclosed  in  fine  brass  gauze  or  screen  to  prevent  them  from 
igniting  any  gasoline  vapor  which  might  be  present  under  the 
hood  of  the  engine. 

6.  When  we  wish  to  stop  the  magneto  we  cannot  use  a 
clutch  to  disconnect  it  so  it  will  no  longer  turn,  because  when 
we  let  the  clutch  in  or  connect  the  magneto  up  again,  it  would 
be  out  of  step,  or  out  of  time  with  the  motor.  The  method 
for  stopping  the  magneto  is  to  “cut  out”  the  breaker.  In 
other  words,  we  take  the  primary  circuit  and  connect  it  to  the 
ground  before  it  reaches  the  breaker.  In  this  way,  the  primary 
current  is  not  broken  or  ruptured  by  the  breaker,  and  therefore 
we  get  practically  no  secondary  current. 

7.  In  the  Bosch  high  tension  magneto  the  armature  gener- 
ates two  waves  of  primary  current  per  revolution.  It  is 
necessary  to  have  the  break  in  the  primary  occur  at  the  time 
when  the  current  is  the  strongest,  and  this  will  mean  at  the 
highest  point  of  the  two  waves  I have  referred  to.  In  other 
words,  the  correct  time  to  have  the  break  occur  is  when  the 
armature  is  just  leaving  the  pole  pieces  on  the  ends  of  the 
magnets.  The  exact  measurement  between  the  armature  and 
pole  pieces  varies  with  different  magnetos  for  engines  of  dif- 
ferent number  of  cylinders,  but  ordinarily  this  position  of  the 
armature  occurs  when  there  is  an  air  space  of  one-eighth 
of  an  inch  between  the  armature  and  the  pole  pieces.  As 
airplane  motors  are  run  most  of  the  time  at  full  speed  there  is 
seldom  any  necessity  for  changing  the  time  of  the  spark,  but 
on  some  machines  the  spark  is  retarded  to  make  the  cranking 


50 

of  the  motor  safer.  In  all  ignition  systems,  the  spark  is 
advanced  or  retarded  by  changing  the  time  of  the  break  in  the 
primary  circuit.  In  the  high  tension  magneto  this  is  accom- 
plished by  shifting  the  cams  that  operate  the  breaker.  In  the 
Bosch  magneto,  if  the  breaker  housing  that  carries  these  cams 
is  in  the  fully  advanced  position,  the  break  will  occur  with  the 
armature  in  the  ideal  position  that  I have  spoken  of,  but  if 
we  retard  the  spark  the  break  occurs  at  a time  when  the 
armature  has  moved  away  from  the  pole  pieces  a considerable 
distance,  perhaps  half  or  three-quarters  of  an  inch.  The 
result  is  that  this  type  of  magneto  gives  the  strongest  spark 
in  the  full  advance  position  and  a weaker  spark  in  the  full  re- 
tarded position.  Sometimes  the  spark  in  this  position  is  so 
weak  that  the  motor  cannot  be  started  on  it.  It  is  therefore 
well  to  start  the  engine  with  the  spark  as  far  advanced  as  we 
can  have  it  without  making  the  motor  kick  back.  Some 
magnetos  overcome  this  difficulty  of  having  a weak  spark  in 
the  retarded  position  by  rocking  the  magnets  with  the  breaker 
housing,  or  in  other  words,  moving  the  pole  pieces  at  the  same 
time  we  move  the  breaker  cams,  so  that  the  break  will  occur 
with  the  armature  in  the  ideal  position  anywhere  from  full 
advance  to  full  retard.  The  Eismann,  Mea  and  Dixie  mag- 
netos are  of  this  type.  Some  magnetos  are  arranged  so  that 
there  are  four  positions  of  the  armature  or  rotor,  per  revolu- 
tion, in  which  conditions  are  right  to  have  the  break  occur. 
These  magnetos  then  deliver  four  sparks  per  revolution  (of 
the  magneto),  and  such  magnetos  run  at  one-half  the  speed  of 
the  other  types. 

Care  of  the  Magneto. 

8.  First  of  all,  do  not  over-oil  the  magneto.  Clean  oil 
would  not  do  very  much  harm  in  a magneto,  but  oil  as  we  know 
it  around  a motor  is  never  clean.  It  always  contains  some 
carbon  or  metal  particles  which  make  it  a fair  conductor  of 
electricity,  and  for  this  reason  it  can  short-circuit  a magneto. 
Also  if  we  had  oil  between  the  breaker  points  it  would  string 
out  when  the  points  separate  and  supply  a path  for  the  cur- 
rent to  flow  through,  thus  preventing  a quick  and  complete 
break  in  the  primary  circuit.  The  breaker  mechanism  of  all 
magnetos  is  arranged  to  run  without  oil  and  should  not  be 
oiled.  The  ball-bearings  on  the  armature  and  distributor 
bearings  must  be  oiled,  but  only  every  thousand  miles  or  once 
a week,  and  then  with  a few  drops  of  high-grade,  light  oil.  In 
cleaning  the  distributor  we  sometimes  find  the  contacts  or 
the  segments  blackened  and  it  becomes  necessary  to  brighten 


51 

them  up.  This  should  be  done  with  brass  polish,  pumice 
stone  powder,  whiting  or  crocus  cloth,  but  never  with  sand 
paper  or  emery  cloth  no  matter  how  fine  they  are  because  the 
grit  from  these  cloths  would  become  embedded  in  the  soft 
insulation  material  of  the  distributor  and  cause  scratching  or 
tearing.  In  many  magnetos,  there  are  numerous  carbon 
brushes  which  are  apt  to  become  glazed,  and  when  found  in 
such  condition  they  should  have  the  glaze  scraped  off,  leaving 
them  dull.  All  these  carbon  brushes  have  light  springs  behind 
them.  Never  stretch  these  springs  to  cause  a firmer  contact 
because  it  is  unnecessary,  and  if  the  contact  is  made  firmer 
the  carbon  brush  will  wear  faster,  spreading  a streak  of  carbon 
powder  around  the  part  it  touches,  and  wull  be  likely  to  short- 
circuit  the  magneto  in  this  way.  After  cleaning  the  distribu- 
tor, oil  can  be  rubbed  around  it  with  the  finger;  then  the  oil 
should  be  wiped  lightly  with  a clean  rag,  leaving  only  an  oily 
appearance  which  is  about  the  right  condition.  If  too  much 
oil  is  left  in  the  distributor,  it  will  form  a paste  with  the  powder 
from  the  carbon  brushes  and  conduct  the  current  from  one 
contact  to  the  next.  If  the  distributor  is  left  too  dry,  as  it 
would  be  after  washing  with  gasoline,  cutting  and  tearing 
would  result.  If  the  magneto  is  driven  by  a gear  direct  on  the 
armature  shaft,  great  care  must  be  taken  to  be  sure  that  the 
gears  mesh  properly.  If  the  gears  mesh  too  loosely  they  can 
jerk  back  and  forth  and  strain  the  magneto,  but  if  the  gears 
mesh  too  tightly  the  bearings  on  the  armature  shaft  of  the 
magneto  will  be  ruined  in  a very  short  time,  putting  the  mag- 
neto out  of  commission.  Gear  teeth  should  never  “bottom” — 
that  is,  the  points  of  the  teeth  should  never  touch  the  bottoms 
of  the  spaces  in  the  other  gear,  but  a clearance  of  from  1/64 
to  1/32  of  an  inch  must  be  left  between  the  gears. 

9.  When  inspecting  or  cleaning  a magneto  never  remove  the 
end  plates  which  carry  the  armature  bearings  because  they  are 
accurately  fitted  to  hold  the  armature  just  .002”  from  the  pole 
pieces  on  all  sides.  Never  remove  the  magnets  from  the 
magneto  or  the  armature,  because  if  this  is  done,  some  of  the 
magnetism  or  “pull”  will  be  lost  from  the  magnets.  These 
two  operations  should  only  be  performed  by  a well-equipped 
magneto  expert.  It  is  not  necessary  to  do  these  things  in  the 
field. 

Testing  the  Magneto. 

10.  First,  suppose  we  have  the  magneto  on  the  bench.  By 
taking  hold  of  the  magneto  shaft  where  the  coupling  or  gear 
would  go,  turn  the  magneto  in  its  proper  direction  (usually 


52 

indicated  by  an  arrow  on  the  magneto).  Notice  how  much 
pull  or  resistance  it  offers  as  you  turn  it  over.  This  should 
feel  a good  deal  like  the  compression  in  a motor;  and  by  feeling 
a magneto  in  this  way  which  is  known  to  be  up  to  full  strength, 
and  then  feeling  your  magneto,  you  can  form  an  idea  as  to  the 
condition  or  strength  of  the  magnets.  If  the  magneto  is  on  the 
engine  and  we  wish  to  test  it  to  see  if  it  throws  a spark,  the 
best  place  to  test  it  will  be  from  the  collector  brush  (which 
collects  the  secondary  current  from  the  armature)  and  make  a 
spark  jump  from  this  brush-holder  to  the  frame  of  the  engine 
by  holding  a screw-driver  against  the  engine  and  leaning  it  so 
it  passes  close  to  the  brush-holder.  The  magneto  should 
throw  a spark  at  least  an  eighth  of  an  inch  while  the  engine  is 
being  cranked  by  hand,  but  we  must  remember  one  important 
thing.  Whenever  conditions  are  right  for  the  magneto  to 
deliver  a spark  at  the  collector  brush  it  will  also  deliver  sparks 
to  the  spark  plugs  in  the  cylinders.  This  would  make  the 
cranking  of  the  motor  dangerous;  therefore,  we  must  either 
remove  all  the  wires  from  the  spark  plugs,  which  would  be 
quite  a job,  or  the  more  convenient  methbd  is  td  remove  the 
connection  between  the  collector  brush  and  the  distributor,  in 
order  to  safeguard  the  man  who  is  cranking.  Also  remember 
that  the  switch  must  be  open  when  we  are  testing  the  magneto. 

11.  If  we  should  fail  to  find  the  spark  the  first  thing  to  do  is 
to  remove  the  ground  wire  or  short-circuit  wire  from  the  mag- 
neto, because  if  there  is  any  defect  or  short-circuit  in  the 
ground  wire  or  in  the  switch,  it  would  have  the  effect  of  con- 
tinually shorting  the  magneto  even  when  the  switch  is  open. 
So  if  we  get  a spark  only  after  disconnecting  the  ground  wire, 
we  can  be  sure  that  the  ground  wire  in  some  way  made  contact 
with  the  frame  of  the  engine  when  the  switch  is  open. 

12.  Let  us  clearly  understand  that  when  the  switch  is 
“closed”  the  primary  current  from  the  magneto  flows  through 
the  switch  into  the  ground  or  frame  of  the  engine  instead  of 
flowing  through  the  breaker  of  the  magneto  and  therefore  we 
get  no  spark  in  the  cylinders  and  we  say  that  the  motor  is  safe. 
When  the  switch  is  “open”  that  means  that  there  is  no  path 
for  the  primary  current  to  flow  directly  to  the  ground  and  it 
must  flow  through  the  breaker.  In  this  way  a spark  will  be 
delivered.  But  should  our  ground  wire  chafe  against  some 
metal  part  in  the  airplane  so  that  the  wire  itself  makes  actual 
contact  with  this  metal,  the  condition  would  be  the  same  as 
having  the  switch  closed  all  the  time.  If  we  test  the  magneto 
and  find  no  spark,  the  next  thing  we  should  examine  after 
testing  our  ground  wire  should  be  the  breaker  points.  See 


I 


53 

that  they  open  the  correct  distance  for  that  particular  kind 
of  a magneto.  (Note. — The  Bosch  magneto  requires  .015" 
gap  between  the  breaker  points;  the  Berling  requires  .016"  to 
.20"  and  the  Dixie  requires  .020".)  The  breaker  points 
must  open  the  correct  gap,  no  more  and  no  less.  Any  change 
in  the  adjustment  of  the  breaker  will  cause  the  break  to  occur 
with  the  armature  a different  distance  from  the  pole  pieces,  and 
may  have  considerable  effect  on  the  working  of  the  magneto. 
The  breaker  points  should  be  clean  and  smooth.  If  we  find 
them  badly  burned  we  can  true  them  up  with  a jeweler’s 
file  and  re-adjust  them  so  that  they  open  the  correct  distance 
or  separate  the  correct  distance.  However,  if  they  are  badly 
burned  we  should  endeavor  to  get  another  magneto,  because 
the  fact  that  they  are  burning  shows  that  the  condenser  is 
not  working  properly. 

13.  I have  found  by  experience  that  we  can  tell  more  about 
the  actual  strength  of  a magneto  by  the  appearance  of  the 
spark  plug  points  than  by  any  other  means  we  have  on  the 
field.  The  correct  distance  between  spark  plug  points  for 
Bosch  magneto  and  Dixie  magneto  is  1/64"  or  .015";  for  a 
Berling  magneto,  .030".  As  long  as  these  spark  plugs  are 
properly  adjusted,  if  the  magneto  is  up  to  full  strength  the  heat 
of  the  spark  will  burn  these  points  to  a whitish  appearance 
between  the  tips,  and  when  the  magneto  begins  to  get  weak 
from  any  cause,  it  will  fail  to  burn  the  plugs  in  this  manner. 
Also  if  the  spark  plug  points  are  too  wide  apart  this  whitish 
appearance  will  not  be  noted.  I have  found  that  by  carefully 
watching  the  spark  plug  points,  I have  often  been  able  to 
remove  a magneto  which  was  becoming  weak  before  the 
engine  had  missed  a shot,  in  this  way  preventing  trouble.  Also, 
suppose  that  in  an  eight-cylinder  motor  seven  of  our  spark 
plugs  show  this  white  appearance  between  the  tips  and  the 
eighth  does  not.  If  all  the  plug  points  are  adjusted  equally, 
this  will  indicate  that  the  insulation  in  this  spark  plug  is 
defective  and  usually  if  you  will  break  open  such  a spark 
plug  you  will  find  a crack  in  the  porcelain  which  will  usually 
be  blackened  by  smoke.  This  will  prove  that  you  were  right 
in  removing  the  plug. 

14.  Remember  that  in  ignition  work,  it  is  the  heat  of  the 
spark,  rather  than  the  size  of  the  spark,  which  counts.  If  you 
are  out  somewhere  with  a motor  and  find  that  the  magneto  is 
becoming  weak  and  have  no  replacement  for  it,  you  can  gener- 
ally cause  it  to  burn  the  spark  plug  points  white  by  closing  the 
points  together  slightly;  for  instance,  closing  them  in  to  .010"; 
in  this  way  you  will  be  able  to  keep  the  magneto  in  commission 


54 

for  several  hours  longer,  sometimes  six  or  ten  hours  longer. 
Another  method  of  keeping  the  magneto  in  commission  when 
it  begins  to  weaken  is  to  retard  the  breaker  housing  slightly. 
This  applies  to  the  Bosch  and  Berling  magnetos  and  will  often 
enable  you  to  get  home  with  a motor  or  machine.  You  must 
not  retard  the  spark  too  far  or  overheating  of  the  motor  will 
result. 

15.  Remember  that  magnetos  give  trouble  mostly  at  high  speed , 
not  at  low  speed , and  when  you  can  get  a spark  from  a magneto 
by  testing  it  in  the  manner  I have  described  the  motor  will 
usually  start  well  and  run  all  right  up  to  about  one-third  of  its 
speed,  but  may  not  run  at  all  at  high  speed.  In  other  words, 
a magneto  will  start  a motor  unless  it  is  completely  “knocked 
out.”  When  ordering  a new  magneto,  it  is  necessary  to  state 
make,  and  type  of  magneto,  and  number  of  cylinders  on  the 
motor.  Also  it  is  necessary  to  state  whether  you  require 
a “clockwise”  or  “anti-clockwise”  magneto.  A magneto  is 
said  to  revolve  “clockwise”  or  “anti-clockwise”  as  seen  when 
looking  at  the  gear  or  coupling  end  of  the  armature  or  rotor 
shaft;  this  does  not  refer  to  the  direction  of  rotation  of  the 
distributor. 

Timing  the  Magneto. 

16.  Timing  the  magneto  consists  of  three  operations.  First, 
we  must  place  the  motor  or  crank  shaft  in  the  proper  position 
for  the  spark  to  occur.  Second,  we  must  place  the  magneto  in 
the  position  where  it  is  delivering  a spark,  and  third,  we  must 
mesh  the  gears  or  connect  the  coupling. 

Placing  the  Motor. 

17.  First  of  all,  it  is  usual  to  time  a magneto  for  the  No.  1 
cylinder  of  a motor.  This  is  not  necessary,  but  it  is  the  con- 
ventional practice.  Therefore,  we  will  place  the  piston  of 
No.  1 cylinder  on  the  right  stroke.  Naturally  the  spark  must 
occur  on  the  compression  stroke,  the  only  stroke  when  there 
is  gas  in  the  cylinder  and  compressed  ready  to  fire.  Now  the  ■- 
question  is,  how  shall  we  know  when  No.  1 piston  is  on  the 
compression  stroke?  If  we  will  turn  the  engine  over  in  the 
proper  direction  or  in  the  direction  in  which  it  runs  until  we 
see  the  inlet  valve  open,  we  know  that  we  are  at  the  beginning 
of  the  suction  stroke  and  when  we  see  the  inlet  valve  close  we 
will  know  that  the  piston  is  at  the  end  of  the  suction  stroke  I 
and  at  the  beginning  of  the  compression  stroke.  Now,  if  we  I 
will  put  a rod  or  screw  driver  down  through  the  spark  plug 
hole  on  the  head  of  the  piston  we  can  follow  the  piston  as  it 


55 

comes  up  on  the  compression  stroke  and  find  the  top  center  or 
highest  point  of  the  piston  travel.  It  is  best  to  find  this  point 
accurately , to  measure  from. 

18.  Airplane  motors  are  usually  timed  in  full  advanced 
position  because  they  are  seldom  retarded.  In  other  words, 
the  advanced  position  is  the  most  important.  Now,  the  ques- 
tion is  how  much  advance  shall  we  give  this  motor,  or,  how  far 
before  top  center  shall  the  spark  occur?  The  amount  of  ad- 
vance depends  on  the  speed  at  which  the  motor  runs,  also  on 
the  type  of  combustion  chamber,  whether  the  motor  is  of  high 
or  low  compression,  and  whether  there  is  one  or  two  spark 
plugs  per  cylinder.  The  faster  the  motor  runs  the  greater 
advance  we  must  give  the  spark  or  the  earlier  the  spark  must 
occur.  If  we  have  a compact  combustion  chamber,  of  the 
valve  in  the  head  type,  or  if  we  have  two  sparks  per  cylinder 
simultaneously,  less  time  will  be  required  for  the  flame  to  travel 
through  the  entire  charge  and  therefore  less  advance  will  be 
required.  An  experienced  motor  man  can  judge  fairly  closely 
where  the  spark  should  occur  on  any  motor  if  he  knows  these 
conditions.  But  most  men  will  do  well  to  refer  to  the  instruc- 
tion book  for  their  particular  motor  or  to  get  the  information 
from  a superior.  Whenever  we  disassemble  a motor  or  remove 
a magneto  we  must  make  it  a point  to  determine  by  measure- 
ment where  the  spark  occurs  so  that  we  will  have  this  infor- 
mation for  that  particular  motor. 

19.  As  airplane  motors  seldom  have  fly  wheels  the  spark 
timing  is  generally  given  in  inches  of  piston  travel.  Instead 
of  saying  the  spark  should  occur  so  many  degrees  before  top 
center,  it  is  usual  to  say  the  spark  must  occur  at  a certain 
fraction  of  an  inch  before  top  center,  meaning  for  instance,  a 
quarter  of  an  inch  before  the  piston  reaches  the  top  of  its 
stroke.  Then  to  complete  the  process  of  placing  the  motor, 
suppose  our  instructions  for  the  motor  we  are  working  on  are 
to  set  the  spark  5/16"  before  top  center.  We  have  already 
placed  our  piston  at  the  top  center  of  the  compression  stroke. 
We  will  now  take  a scale  or  rule  and  measure  5/16"  upon  our 
screw  driver  or  rod  and  make  a mark.  Then  we  will  back  the 
motor  until  the  piston  has  gone  down  this  5 /16".  The  piston 
in  No.  1 cylinder  is  now  in  the  correct  position  and  on  the 
right  stroke  for  the  spark  to  occur. 

Placing  the  Magneto. 

20.  We  must  first  decide  which  distributor  contact  we  will 
call  No.  1.  Frequently,  a figure  "1"  will  appear  in  a little 
window  injdie  distributor  cover  when  this  contact  is  made. 


56 

The  breaker  breaks  once  for  each  contact  of  the  distributor. 
Turn  the  magneto  until  the  distributor  brush  is  on  No.  1 con- 
tact. We  will  now  turn  the  magneto  carefully  a few  degrees 
until  the  breaker  points  just  begin  to  separate.  Just  as  soon  as 
we  can  see  that  the  points  are  separating,  that  is  when  the 
spark  occurs.  Another  method  is  to  previously  insert  a cigar- 
ette paper  between  the  breaker  points.  This  kind  of  paper  is 
only  .001"  thick  and  when  it  will  slip  out  it  will  show  us  when 
the  points  are  separated  just  that  amount.  That  is  perhaps 
the  most  accurate  way  to  "get  the  break."  We  must  also  be 
sure  that  the  breaker  housing  is  in  the  advanced  position 
because  we  have  no  desire  ever  to  make  the  spark  occur  earlier 
than  5 /16"  before  top  center  in  this  particular  motor.  Now 
placing  the  distributor  and  breaker  points  we  can  mesh  the 
gear  or  connect  the  coupling.  Then  our  magneto  will  be  timed. 
Sometimes,  when  the  magneto  has  been  accurately  placed,  we 
find  that  the  gears  will  not  quite  mesh  but  require  turning  in 
one  direction  or  the  other  perhaps  half  a tooth.  Many  mag- 
netos are  provided  with  adjustable  couplings  to  take  care  of  such 
a situation,  but  if  our  magneto  is  not  so  provided,  we  will  have 
to  decide  whether  we  prefer  to  have  the  spark  occur  half  a 
tooth  earlier  or  half  a tooth  later.  If  the  motor  is  using  a 
rather  large  amount  of  advance  perhaps  half  a tooth  later 
would  be  better;  and  if  our  motor  happens  to  carry  rather 
little  spark  advance,  a half  tooth  earlier  would  do  no  harm. 
Where  two  magnetos  are  used,  they  must  be  timed  exactly 
alike  or  "synchronized." 

Wiring  up  the  Magneto. 

21.  The  cylinders  receive  the  sparks  in  the  same  order  that 
they  get  their  gas  through  the  inlet  valves.  When  the 
designer  of  a motor  designs  the  cam-shaft,  he  arranges  it  to 
open  the  inlet  valves  and  deliver  gas  to  the  cylinders  in  some 
certain  order.  For  example,  a 4-cylinder  engine  never  fires  in 
numerical  order.  It  will  fire  either  1-2-4-3  or  1-3-4-2.  If  it 
should  fire  1-2-3-4  it  would  have  a great  amount  of  vibration 
and  the  strains  on  the  crank  shaft  would  be  excessive.  It  is 
important  then  for  the  motor  to  have  a certain  firing  order  to 
make  it  run  smoothly  and  also  to  make  it  draw  gas  through 
the  inlet  manifold  without  jerking  the  gas  back  and  forth  or 
causing  it  to  reciprocate.  If  the  engine  fires  in  the  same  order 
in  which  the  inlet  valves  open,  obviously  the  way  to  find  out 
the  firing  order  of  any  motor  is  to  crank  it  over  and  note 
in  which  order  the  inlet  valves  open.  For  example,  we  have 


57 

a six-cylinder  motor.  We  will  start  with  No.  1 cylinder  and 
turn  the  engine  over  until  No.  1 inlet  valve  is  just  opening. 
Then  we  will  put  down  the  figure  1 on  our  paper.  Now  we 
can  turn  the  motor  over  a few  degrees  and  see  which  inlet  valve 
opens  next.  It  may  be  No.  4.  Then  the  next  may  be  No. 
3,  6,  2 and  5.  Remember  that  we  must  not  take  for  granted 
that  all  motors  of  one  make  have  the  same  firing  order  because 
some  may  be  right  hand  motors  and  some  left  (normal  or 
anti-normal  rotation),  and  also  the  manufacturer  may  have 
found  that  he  could  improve  the  running  of  his  motor  by  chang- 
ing the  firing  order.  The  point  I want  to  make  clear  is  this: 
Remember  the  method  of  finding  the  firing  order  rather  than  try- 
ing to  remember  the  firing  order.  It  is  but  a few  minutes 
work  to  turn  the  motor  over  and  note  the  actual  firing  order, 
and  by  doing  so  you  will  save  yourself  trouble  some  time. 
The  magneto  delivers  the  sparks  in  numerical  order,  1,  2,  3,  4, 
etc.,  because  as  the  distributor  brush  moves  around  it  touches 
one  contact  after  the  other,  but  try  to  think  of  the  distributor 
terminals  not  as  No.  1,  2,  3,  but  as  first,  second,  third,  etc. 

22.  In  timing  the  magneto,  we  time  it  for  No.  1 cylinder. 
So  we  can  connect  No.  1 terminal  on  the  distributor  to  No.  1 
spark  plug,  and  the  second  terminal  on  the  distributor  will 
connect  to  the  second  cylinder  in  the  firing  order,  and  the  third 
distributor  terminal  to  the  third  in  the  firing  order,  and  so  on. 


Magneto 1st  2d  3d  4th 

Cylinder  Nos 1 4 3 2 


23.  Above  all,  in  timing  a magneto,  do  not  forget  to  place 
the  engine  on  the  right  stroke.  Before  timing  the  cam  shaft, 
the  strokes  on  the  pistons  were  merely  up  strokes,  and  down 
strokes,  but  since  timing  the  cam  shaft,  each  stroke  has  a name, 
viz.,  suction  stroke,  compression  stroke,  working  stroke,  and 
exhaust  stroke.  The  spark  must  occur  near  the  top  of  the 
compression  stroke,  when  there  is  gas  in  the  cylinder  and  it  is 
compressed  ready  to  fire.  This  only  occurs  every  alternate 
time  the  piston  is  up.  If  the  spark  occurred  on  the  other  top 
center  no  explosion  would  occur  as  there  would  be  no  com- 
pression and  no  gas  in  the  cylinder,  and  if  you  should  go  so  far 
as  to  start  the  motor,  you  would  not  get  so  much  as  a “shot” 
out  of  it. 


LECTURE  VI. 

SPARK  PLUGS. 

1.  A spark  plug  is  composed  of  an  outer  steel  shell  which 
screws  in  the  cylinder  and  a core  of  insulation  material  usually 
made  of  some  form  of  porcelain  with  a wire  running  down  the 
center.  This  wire  is  known  as  the  central  electrode.  The  high 
pressure  current  or  “spark”  from  our  magneto  is  delivered  to 
this  central  electrode  and  jumps  from  its  inner  end  to  some  point 
arranged  on  the  steel  shell  of  the  plug,  which,  being  in  contact 
with  the  cylinder,  is  grounded.  When  mica  insulation  is  used 
it  has  a tendency  to  become  oil-soaked  in  time,  and  with  the 
porcelain  insulations  there  is  danger  of  their  cracking  from 
great  heat  or  rapid  changes  in  temperature.  The  electrodes 
are  usually  made  of  some  metal  which  does  not  scale  easily 
under  high  temperatures.  The  scale  would  be  in  the  nature 
of  an  insulator  if  it  could  form.  Platinum,  iridium  and  nickel 
alloys  are  generally  used  for  electrodes.  The  proper  adjust- 
ment of  the  gap  between  the  electrodes  at  the  inner  end  of  the 
spark  plugs  should  be  accurately  made.  Most  of  the  high 
tension  magnetos  require  a gap  of  1/64"  or  .015"  and  nearly 
all  modern  battery  systems  require  a gap  of  about  .030" 
or  approximately  1/32".  With  high  tension  magnetos  if 
the  spark  plug  points  are  set  too  close  together  the  heat  of  the 
spark  is  greater  than  when  the  points  are  far  apart.  If  the 
spark  plug  points  are  too  close  the  heat  of  the  spark  will  be 
sufficient  to  fuse  or  melt  the  tip  and  little  globules  or  bubbles 
of  metal  are  formed  on  the  tips  which  are  liable  to  connect  or 
short-circuit  them,  and  if  the  spark  plug  points  are  too  far 
apart  it  will  be  difficult  for  the  magneto  to  throw  a spark 
across  them  when  the  engine  is  running  at  low  speed  and  the 
magneto  also  running  at  low  speed.  Because  a spark  will 
jump  3/16"  outside  the  cylinder  does  not  mean  that  it  will 
jump  that  far  inside.  Remember  that  the  spark  inside  the 
cylinder  occurs  while  the  gas  is  compressed  to  90  or  100 
pounds  to  the  square  inch,  and  under  these  conditions  can 
jump  only  about  one-third  as  far  as  it  can  jump  outside  the 
cylinder  where  the  air  is  at  atmospheric  pressure. 

2.  I have  pointed  out  that  it  is  the  heat  of  the  spark  rather 
than  the  size  of  the  spark  which  counts  and  also  that  the 
magneto  makes  a hotter  spark  when  the  points  are  close  than 
when  they  are  far  apart,  and  there  are  other  arguments  in  favor 

(58) 


59 

of  having  the  plug  points  close  together.  If  the  plug  points 
are  far  apart,  the  resistance  to  the  flow  of  the  current  will  be 
high  and  it  will  not  require  very  much  dirt  or  oil  on  the  surface 
of  the  insulation  to  provide  an  easier  path  for  the  current 
than  jumping  the  gap.  In  other  words,  with  the  plug  points 
farther  apart,  the  plug  will  be  more  easily  short-circuited.  I 
have  often  seen  spark  plugs  with  cracked  insulation  begin  to 
miss  at  high  speed  and  by  taking  those  plugs  and  putting  the 
points  closer  together,  thus  reducing  the  resistance  at  the 
points,  the  current  would  once  more  jump  the  gap  and  the 
cylinder  continue  to  fire.  Under  these  circumstances,  the 
plug  would  fire  until  sufficient  carbon  accumulated  in  the  crack 
in  the  porcelain  to  provide  an  easier  path  for  the  current  than 
the  reduced  gap  in  the  plug.  It  is  always  best  to  adjust  spark 
plug  points  accurately  to  a gage,  because  in  a multi-cylinder 
motor  the  heat  of  the  spark  should  be  unifo  rm  in  all  cylinders. 

3.  Oceasionally  we  find  a motor,  or  one  cylinder  of  a motor, 
which  floods  with  oil  and  continually  fouls  or  short-circuits 
spark  plugs.  Under  these  conditions,  a single  pointed  spark 
plug  should  be  used,  or  if  you  are  using  a three-point  plug  in  all 
the  cylinders,  break  off  two  of  the  points  in  this  particular 
cylinder.  The  reason  for  this  is  that  in  a three-point  plug 
while  the  spark  is  jumping  between  a certain  pair  of  points 
and  burning  them  clean,  oil  and  carbon  are  accumulating 
between  the  points  where  no  spark  is  occurring.  In  a single 
point  plug,  there  is  a strong  tendency  for  the  spark  to  keep  the 
points  clean,  due  to  its  great  heat.  Also  the  points  should  be 
close  together  and  in  this  way  the  current  will  be  able  to  jump 
through  the  oil  instead  of  flowing  through  it.  In  spark  plugs 
it  is  a great  problem  to  carry  the  heat  away  from  the  central 
electrode  fast  enough  so  that  this  part  will  not  become  red  hot 
or  incandescent.  In  some  spark  plugs  the  points  do  become 
red  hot  and  cause  pre-ignition.  This  will  usually  show  up  as 
follows:  You  can  start  your  motor  on  a test  block  or  in  a 
machine  and  it  will  run  smoothly  for  two  or  three  or  possibly 
ten  or  fifteen  minutes,  and  then  begin  to  jerk  or  jar  a little  bit. 
In  many  cases,  it  will  begin  to  shake  steadily  and  regularly 
appearing  as  excessive  vibration.  Usually  the  speed  of  the 
motor  will  be  slightly  reduced.  The  thing  to  do  in  a case  like 
this  is  to  experiment  with  some  kind  of  spark  plug  having 
shorter  and  thicker  electrodes. 

Care  of  Spark  Plugs. 

4.  When  we  remove  spark  plugs  for  cleaning  we  should 
gently  scrape  the  carbon  away,  being  careful  not  to  wedge 


60 

anything  between  the  shell  of  the  plug  and  the  insulation, 
because  we  might  crack  the  insulation  in  this  manner.  Adjust 
the  spark  plugs  hy  your  gage  and  note  the  appearance  of  the 
points.  They  should  be  burned  whitish  in  color  between  the 
points.  If  they  are  not,  it  probably  indicates  that  the  points 
are  set  too  far  part  or  that  the  magneto  is  weak.  If  one  plug 
in  the  set  fails  to  show  this  whitish  appearance,  it  should  be 
replaced,  because  the  insulation  is  probably  defective.  If  you 
find  small,  metallic  beads  or  bubbles  on  the  points,  they 
should  be  adjusted  .002"  or  .003"  farther  apart. 

5.  It  is  well  to  use  new  spark  plug  gaskets  frequently.  By 
so  doing  it  will  be  unnecessary  to  screw  the  plug  in  very  tightly 
and  the  danger  of  straining  or  cracking  the  insulation  by  pull- 
ing hard  on  the  shell  of  the  plug  with  your  wrench  will  be 
greatly  reduced.  Never  drop  spark  plugs,  because  there  is 
danger  of  cracking  the  porcelain;  and  always  shake  them  to 
see  if  you  hear  any  rattle  in  the  porcelain.  Unfortunately 
there  is  often  no  way  in  which  we  can  detect  the  crack  in  the 
plug  unless  it  is  by  noting  that  the  spark  fails  to  burn  the  points 
white  or  that  the  cylinder  mises  at  high  speed.  It  is  an  excel- 
lent plan  to  put  a few  drops  of  oil  on  the  spark  plug  thread  be- 
fore screwing  it  into  the  cylinder.  If  you  ever  have  one  stick 
in  a cylinder,  you  will  never  fail  to  do  this. 


LECTURE  VII. 

INSPECTION  OR  “PREVENTION”  OF  TROUBLE. 

1.  In  airplane  motor  work,  if  the  motor  fails,  the  aviator 
usually  fails  to  accomplish  whatever  he  started  out  to  do. 
This  means  that  it  is  important  to  prevent  trouble  by  means  of 
careful  planning,  thorough  inspection  and  forethought,  rather 
than  to  wait  until  the  trouble  occurs  and  then  figure  out  a way 
to  fix  it.  Inspect  in  a systematic  way.  Begin  by  inspecting 
the  gas  feed.  To  do  this,  first  examine  the  gas  tank  and  see 
that  there  is  a good  air  vent,  and  see  that  the  air  vent  is  not 
plugged  up,  because  gasoline  will  not  flow  from  the  tank  very 
long  unless  air  can  get  into  take  the  place  of  the  gasoline. 
Next,  test  the  flow  from  the  pipe  where  it  connects  with  the 
carburetor.  Clean  the  settlers  and  strainers  in  the  gasoline 
line.  Clean  the  carburetor  thoroughly,  including  the  jets. 
Then  examine  the  intake  manifolds,  for  air  leaks  or  loose  joints. 
It  is  important  to  examine  the  throttle  controls.  The  best 


61 

way  is  to  have  someone  sit  in  the  pilot’s  seat  and  operate  the 
throttle,  then  stand  where  you  can  see  the  carburetor  and  be 
sure  that  when  he  operates  the  throttle  that  it  opens  all  the 
way  and  be  very  sure  that  when  he  closes  the  throttle  it  closes 
all  the  way.  This  is  exceedingly  important  because  it  is  quite 
possible  to  install  a new  motor  or  carburetor  in  a machine  and 
the  throttle  controls  may  not  fit.  The  result  may  be  that 
the  throttle  will  not  come  anywhere  near  closing.  Then  when 
the  mechanic  cranks  the  engine,  the  machine  may  start  ahead 
so  that  the  propeller  will  strike  him.  Next  let  us  inspect  the 
ignition.  See  that  the  ground  wire  or  short  circuit-wire  con- 
nections are  tight  and  examine  its  condition  throughout  its 
entire  length.  See  that  it  does  not  vibrate  or  chafe  against 
any  metal  parts,  because  after  the  insulation  wears  through, 
the  wire  will  short-circuit  against  metal  parts  of  the  machine, 
and  the  effect  will  be  the  same  as  having  the  switch  closed  all 
the  time.  Sometimes  a nail  or  tack  will  have  been  driven 
through  the  ground  wire  accidentally.  This  will  do  the  same 
thing.  See  that  the  connections  of  the  wire  are  firm  and  not 
likely  to  work  loose,  because  if  they  should  work  loose,  the 
pilot  will  have  no  way  to  stop  the  spark  occurring  in  the  cylin- 
ders, making  it  necessary  to  shut  off  the  gasoline  to  stop  the 
motor  and  making  it  dangerous  for  the  man  who  cranks  the 
propeller. 

2.  Examine  the  switch;  see  that  there  are  no  loose  parts. 
Sometimes  trouble  will  occur  from  a strand  coming  loose  on 
a wire  where  it  is  attached  inside  the  switch,  and  these  strands 
may  touch  the  opposite  part  of  the  switch  and  short-circuit  it. 
Sometimes  the  switch  will  become  corroded  so  that  it  will  not 
make  a contact  and  the  motor  will  fail  to  stop  when  we  close 
the  switch,  or  it  might  be  dangerous  to  the  man  cranking  the 
propeller.  Always  remember  that  on  Bosch  and  Berling 
magnetos  and  most  other  makes  when  the  breaker  cap  or  cover 
is  removed  so  that  you  can  see  the  breaker  points  operate,  it 
will  be  dangerous  to  crank  the  motor  because  you  contact  for 
short  circuiting  the  magneto  is  usually  in  this  cap  or  cover. 
Therefore,  if  you  wish  to  crank  the  motor  to  see  how  the 
breaker  points  operate,  always  remove  the  connection  between 
the  distributor  and  the  collector  brush,  or  remove  the  spark 
plugs.  Next,  we  can  clean  the  magneto.  First  clean  the 
breaker  points.  The  breaker  should  not  be  adjusted  unless 
they  are  considerably  wrong,  and  if  possible,  they  should  be 
trued  up  with  a jeweler’s  file  when  readjusted.  The  reason 
is  because  after  the  points  are  adjusted  they  seldom  come  to- 
gether true  and  must  be  squared  up  with  a file.  The  best 


62 

policy  is  to  leave  the  breaker  points  alone  in  most  cases,  but 
if  you  find  them  burned  badly  or  find  that  they  are  badly 
in  need  of  adjustment,  it  must  of  course  be  done.  Clean  the 
distributor;  the  best  way  to  do  this  is  to  wipe  it  with  an 
oily  rag  and  then  wipe  off  all  the  superfluous  oil  with  a dry  rag, 
leaving  it  slightly  oily  looking.  Examine  the  entire  magneto 
from  the  outside  and  see  that  everything  is  tight.  Next  re- 
move the  spark  plugs.  Clean  and  adjust  the  points  and  see 
if  they  are  receiving  the  proper  strength  of  spark.  See  if  they 
are  burning  white.  Be  sure  to  examine  the  plug  for  defect 
or  crack  in  the  insulation.  Examine  the  secondary  wire, 
running  from  the  distributor  to  the  spark  plugs  and  see  that 
they  are  not  chafing  against  any  moving  parts,  such  as  the 
rocker  arms  or  push-rods. 

2.  Remember  that  these  wires  carry  high  pressure  current 
and  any  injury  to  the  insulation  is  likely  to  cause  a leak  or 
short  through  the  frame  of  the  motor.  It  is  well  to  avoid  run- 
ning these  wires  close  to  metal  parts  because  the  rubber,  in 
time,  becomes  cracked  by  the  weather,  and  if  we  have  a foggy 
morning,  making  things  damp,  the  current  may  leak  through 
these  cracks  and  jump  to  the  cylinders  or  ground.  Next  we 
can  inspect  the  valve  operating  gear.  In  high  speed  motors, 
all  parts  of  the  valve  operating  gear  must  work  freely.  There 
must  be  no  rubbing  or  binding.  Remember  that  the  valve 
springs  must  overcome  the  inertia  of  the  heavy  valves  and 
return  them  to  their  seats  in  an  extremely  short  space  of  time. 
When  the  motor  is  running  fast,  they  must  also  overcome  the 
inertia  from  the  push  rods  and  rocker  arms  or  other  parts  of 
the  valve  gear  and  return  them  to  their  normal  positions.  It 
requires  a surprising  amount  of  power  to  do  this,  and  if  there 
is  any  excess  friction  or  binding  in  the  valve  operating  gear, 
the  spring  will  fail  to  do  it,  and  inaccurate  valve  action  will 
result. 

3.  Cam  followers  are  usually  shaped  on  the  end  in  such  a 
way  that  they  must  be  held  in  a certain  position  to  ride  the 
cams  properly.  Frequently  they  are  held  by  some  form  of  set 
screw,  and  therefore  it  should  be  inspected  from  time  to  time 
to  see  if  it  is  wearing.-  If  a cam  follower  should  turn  around 
it  will  usually  ruin  both  the  cam  followers,  and,  what  is  worse, 
the  cam  shaft. 

4.  The  tension  of  the  exhaust  valve  springs  must  be  correct, 
or  they  are  liable  to  become  weakened  due  to  their  great  ten- 
sion in  some  cases,  and  also  due  to  the  fact  that  they  are  sub- 
ject to  considerable  heat  in  some  motors.  In  one  motor  that 
I know  of,  if  an  exhaust  pipe  gasket  blows  out,  the  exhaust 
flame  will  come  in  contact  with  the  exhaust  spring,  thus  draw- 


r>3 

ing  the  temper  from  the  steel  and  allowing  the  spring  to  col- 
lapse. These  springs  are  always  tested  when  the  motor  is 
being  overhauled,  but  sometimes  they  weaken  while  the  motor 
is  in  the  machine,  and  can  usually  be  detected  by  running  the 
motor  at  its  slowest  idling  speed  and  listening.  The  valve 
with  weak  springs  will  usually  suck  open,  or  open  automatically 
during  the  suction  stroke  and  will  usually  make  a buzzing 
sound  as  it  does  this.  Sometimes  it  can  be  detected  by  putting 
the  finger  on  each  exhaust  valve  spring  in  turn  and  the  one 
which  is  sucking  off  its  seat  will  be  vibrating  in  a peculiar 
manner.  When  these  exhaust  valve  springs  become  weak,  air 
will  be  drawn  through  the  valves  during  the  suction  stroke 
and  will  flow  into  the  cylinder  and  also  into  the  intake  mani- 
fold and  spoil  the  mixture  for  all  cylinders  on  that  part  of  the 
manifold.  Its  action  is  very  similar  to  that  of  a loose  joint 
admitting  air  in  the  manifold.  On  the  Curtiss  eight-cylinder 
one  hundred  horse  power  motors  a pull-down  spring  is  used 
for  opening  the  inlet  valve.  This  spring  must  be  at  least  ten 
pounds  heavier  than  the  intake  valve  spring  or  else  the  valve 
will  not  open  properly  and  a “ clicking”  sound  will  be  heard 
while  the  engine  is  running  slowly. 

5.  Then,  next,  we  should  adjust  the  valve  clearance.  The 
designer  of  engines  decides  that  he  wishes  his  valve  to  operate 
with  a certain  clearance.  He  designs  the  cams  to  give  the 
correct  valve  timing  with  this  clearance.  Then  the  engineers 
who  test  the  motor  at  the  factory  find  what  clearance  they 
must  give  the  valves  when  they  are  cold,  in  order  to  have  the 
correct  clearance  when  they  are  hot.  This  clearance  is  pub- 
lished for  all  motors  and  we  should  not  adjust  the  clearance 
on  any  motor  unless  we  know  what  the  correct  clearance  is. 
Having  found  out  what  the  correct  clearance  is  for  our  motor, 
the  next  important  step  is  to  place  the  motor  correctly  for 
adjusting  clearance.  This  must  be  done  for  each  cylinder  in 
the  motor.  The  best  way  to  do  this  is  to  crank  the  motor  in 
direction  of  rotation  until  the  inlet  valve  of  the  No.  1 cylinder 
is  just  closed,  then  turn  the  propeller  90  degrees  further.  This 
will  place  the  cam  followers  on  a neutral  part  of  the  cam  and 
we  can  now  adjust  the  clearance.  This  applies  to  all  four- 
cycle engines,  no  matter  what  the  type.  Always  use  a thick- 
ness gauge  or  “feeler”  for  adjusting  these  clearances.  When 
you  realize  that  a thousandth  of  an  inch  variation  in  valve 
clearance  may  make  four  or  five  degrees  difference  in  the 
timing,  you  will  see  the  importance  of  adjusting  valve  clearance 
accurately  and  uniformly. 


64 

6.  Next,  we  should  try  the  compression  of  the  motor  to  see 
if  all  the  valves  are  seated  properly.  The  best  way  to  do  this 
is  to  remove  a spark  plug  from  each  cylinder.  Then  put  one 
in  one  cylinder  at  a time  so  that  there  will  be  no  doubt  as  to 
which  cylinder  is  on  compression.  One  good  way  is  to  swing 
the  propeller  up  against  the  compression  hard  enough  so  that 
the  compression  will  bounce  it  back  again  several  times,  to  see 
how  many  times  it  can  be  done  before  the  air  has  leaked  by 
the  piston  or  valves  enough  to  let  it  turn  on  over  freely. 

7.  Leaky  exhaust  valves  can  frequently  be  heard  by  listen- 
ing at  the  exhaust  pipes.  Leaky  inlet  valves  can  sometimes 
be  heard  by  listening  in  the  carburetor,  but  when  trying  this, 
have  the  carburetor  or  throttle  partly  open,  because  many 
carburetors  make  a wheezing  sound  when  the  throttle  is  closed. 

8.  Then  examine  each  part,  nut  and  screw,  for  tightness. 
For  instance,  examine  every  cylinder  nut;  then  each  lock-nut 
on  valve  adjustments  and  so  on  in  a systematic  way,  otherwise 
the  loose  ones  will  be  overlooked  nine  times  out  of  ten.  Exam- 
ine the  engine  bed  bolts  to  see  if  they  are  tight  and  locked. 
Examine  the  water  connections.  See  that  there  is  no  chance 
of  the  hose  sucking  shut  in  the  line  between  the  bottom  of  the 
radiator  and  water  pump.  Examine  all  copper  tubing  and 
brass  tubing  carrying  oil,  air,  or  gasoline;  see  that  they  are  free 
from  vibration  or  swinging.  See  that  they  are  not  pinched  or 
likely  to  be;  see  that  they  cannot  be  chafed  by  any  part.  For 
instance,  where  the  pipes  run  through  an  aluminum  bulk-head 
in  the  machine,  sometimes  a hole  is  cut  small  and  then  the  pipe 
will  rub  against  the  edge  of  the  aluminum.  In  this  case,  there 
is  danger  of  the  aluminum  wearing  through  the  copper  pipe 
in  time. 

9.  Inspect  the  Tachometer  drive.  Inspect  the  propeller. 
See  that  it  is  on  tightly  and  locked  securely.  The  entire 
power  output  of  the  engine  is  transmitted  through  the  pro- 
peller hub,  and  unless  it  is  very  tightly  set  on  the  shaft  it  will 
“work”  or  vibrate  and  it  is  liable  to  shear  the  keys  or  loosen 
the  hub.  This  should  be  very  carefully  watched.  A rapidly 
revolving  propeller  has  as  much  ability  to  do  damage  as  a 
large  charge  of  dynamite  and  should  certainly  be  watched. 

10.  Never  “rock”  the  propeller  as  a preparation  to  the 
pull  for  starting.  Many  men  are  hurt  in  this  way.  Place 
your  propeller  where  you  intend  to  start  the  pull,  raise  up  on 
“tip-toes”  and  start  your  pull  strongly.  Remember  you  must 
pick  up  a great  deal  of  speed  in  the  first  few  degrees  in  order  to 
“carry-over”  the  spark.  As  you  finish  the  pull  or  stroke, 
manage  so  that  you  will  be  withdrawing  your  hands.  If  the 


65 

motor  “kicks-back”  never  try  to  resist  it,  simply  withdraw 
your  hands  instantly.  Never  have  tools  in  your  pockets 
while  cranking,  they  may  fly  out  of  your  pockets  and  be 
“batted”  through  your  legs  by  the  propeller. 

11.  The  man  at  the  propeller  and  the  man  at  the  switch 
should  “sound  off”  what  they  are  doing,  so  there  will  be  no 
misunderstanding.  Make  it  a rule  that  the  man  at  the 
switch  will  not  say  “closed”  until  the  switch  is  closed,  because 
if  he  should  say  closed,  and  then  leisurely  reach  over  to  close 
the  switch,  or  make  the  motor  safe,  the  man  at  the  propeller 
might  work  faster  and  pull  the  propeller,  believing  the  motor 
to  be  safe,  and  get  a severe  kick. 

Propeller  Notes. 

A system  for  diagnosing  trouble  or  “trouble  shooting.” 

1 . When  a motor  is  not  working  right,  we  must  first  classify 
our  trouble  or  decide  what  the  motor  is  doing.  For  example, 
does  it  miss,  cut  out,  slow  down,  back-fire,  fail  to  start,  fail  to 
stop,  or  what?  Then,  at  what  speed  does  the  trouble  occur? 
Many  troubles  show  up  only  at  certain  speeds  and  therefore 
we  should  notice  at  what  speed  the  trouble  occurs. 

2.  Next,  if  possible,  notice  what  part  of  the  motor  is  giving 
trouble.  For  example,  on  a “V”  type  motor,  is  the  right  side 
missing  or  the  left  side?  Notice  which  cylinder  is  giving  the 
trouble.  Knowing  which  cylinder  is  giving  trouble,  we  can 
find  the  trouble  comparatively  quickly.  Some  kinds  of 
trouble  can  cause  the  entire  motor  to  give  trouble  but  could 
not  cause  just  one  cylinder  to  give  trouble.  These  could  be 
called  “general”  troubles.  Another  kind  can  cause  one  or 
two  cylinders  to  miss  or  give  trouble  but  could  not  cause  a 
general  trouble.  These  we  call  “local”  troubles. 

3.  For  example,  suppose  we  say  our  motor  is  missing. 
Now,  in  addition  to  noticing  at  what  speed  the  motor  is  miss- 
ing, let  us  note  the  nature  of  the  miss.  For  instance,  is  one  cyl- 
inder missing  occasionally,  one  cylinder  missing  steadily,  one 
side  of  the  motor  cutting  out,  or  the  whole  engine  cutting  out, 
as  it  would  do  if  we  intermittently  pulled  the  switch?  Or  is 
it  a scattering  miss,  which  means  first  one  cylinder  on  one  side 
and  then  another  cylinder  on  the  other  side  missing,  but  not 
regularly?  Suppose  we  decide  that  we  have  one  cylinder  miss- 
ing occasionally.  Some  of  the  likely  causes  of  this  trouble 
would  be,  trouble  from  oil,  slightly  cracked  porcelain  in  spark 
plug,  parts  binding  in  the  valve  action  or  need  of  adjustment 
in  the  valve  clearance;  it  might  be  water  on  the  spark  plug. 


66 

Suppose  we  decide  that  we  have  one  cylinder  missing  steadily. 
This  is  likely  to  be  caused  by  one  of  the  secondary  wires  or 
leads,  running  from  the  distributor  to  the  spark  plug,  being 
disconnected  at  either  end;  a broken  spark  plug  or  a burned 
exhaust  valve  spring,  etc.  Suppose  we  decide  that  one  side  of 
the  motor  is  giving  trouble.  If  this  trouble  occurs  at  high 
speed  it  is  most  likely  to  be  caused  by  magneto  troubles  (this 
can  sometimes  be  remedied  by  slightly  retarding  the  magneto) 
or  it  may  be  an  inlet  valve  stuck  open.  If  the  trouble  occurs 
at  low  speed,  it  is  pretty  sure  to  be  caused  by  an  air  leak  in  the 
inlet  manifold  on  this  side  of  the  engine,  a weak  exhaust  valve 
spring,  which  has  the  same  effect,  or  possibly  an  exhaust  valve 
held  or  stuck  open. 

4.  Magnetos  will  cause  one  side  of  a “V”  type  motor,  or 
every  alternate  cylinder  in  a six  cylinder  motor,  to  cut  out  at 
high  speed,  and  this  is  sometimes  caused  by  the  breaker  points 
opening  a different  distance  on  the  two  cams;  sometimes  by 
the  weakening  of  the  magnets  due  to  vibration  or  other  causes, 
and  sometimes  by  wear  in  the  armature  bearings. 

5.  Suppose  we  decide  that  our  motor  is  “cutting  out.” 
Now  this  is  a “general”  trouble  and  is  likely  to  be  caused  by 
some  troubles  in  the  common  source  of  gas.  For  instance,  in 
the  gas  feed,  by  an  obstruction  in  the  spray  nozzle,  or  water 
in  the  gasoline.  Or  the  trouble  may  be  in  the  source  of  igni- 
tion, not  in  the  distributor  where  the  current  is  handed  out  to 
the  individual  cylinders,  but  probably  in  the  primary  circuit 
in  the  magneto.  It  may  be  in  the  secondary  circuit,  but  if  it 
is,  the  trouble  will  be  between  the  armature  and  the  distributor. 

6.  Occasionally  an  inlet  valve  stuck  open  will  cause  such  a 
violent  back-fire  as  to  make  the  whole  motor  “cut  out,” 
because  when  a back-fire  occurs  it  burns  the  gas  in  the  mani- 
fold and  the  other  cylinders  draw  a mixture  which  is  partly 
burned  gas. 

7.  Now,  we  may  decide  that  we  have  a “scattering  miss” 
in  our  motor.  This  is  harder  to  find,  and  frequently  we  will 
catch  one  cylinder  in  the  act  of  missing  and  believe  we  have 
found  a local  trouble.  This  scattering  miss  is  most  likely  to 
be  caused  by  a weak  magneto,  if  it  occurs  at  high  speed;  but 
it  can  mean  many  things.  It  is  usually  a combination  of 
several  slight  disorders.  If  we  find  that  we  have  a scattering 
miss  at  high  speed,  perhaps  the  best  thing  to  do  is  to  examine 
the  spark  plug  points  to  see  if  they  show  the  proper  heat  or 
strength  of  spark,  and  if  they  do  not,  we  should  put  on  a new 
magneto,  if  possible.  If  a magneto  is  not  obtainable,  we  can 
either  close  the  spark  plug  points  together  a few  thousandths 


67 

of  an  inch,  or  in  extreme  cases,  we  may  slightly  retard  the 
magneto,  which  will  help  in  some  instances.  But  if  we  find 
the  appearance  of  the  spark  plug  such  that  we  may  decide  the 
spark  is  all  right,  then  the  best  thing  is  thoroughly  to  inspect 
the  motor  as  I have  described. 

8.  To  find  out  which  cylinder  is  missing  (if  our  motor  has 
open  exhausts  ports  or  exhaust  pipes  for  each  cylinder), 
the  simplest  way  is  to  hold  a stick  in  front  of  each  exhaust 
in  turn.  In  this  way,  we  can  readily  see  when  a cylinder 
misses  explosions.  Frequently  we  can  watch  the  appearance 
of  the  exhaust  as  it  comes  out  of  the  pipe,  and  at  the  same  in- 
stant that  we  hear  the  motor  miss,  we  may  see  a slight  differ- 
ence in  the  exhaust,  lighter  or  darker  in  one  cylinder.  This 
will  probably  be  the  cylinder  that  is  missing. 

9.  When  seated  in  an  airplane  with  a “V”  type  motor,  it  is 
a good  plan  to  listen  intently  to  the  sounds  coming  from  the 
exhaust  on  one  side  of  the  engine  at  a time.  With  practice, 
you  can  concentrate  on  the  sounds  coming  from  one  side  and 
in  this  way  you  can  tell  a great  deal  more  about  the  operation 
of  the  motor  than  by  listening  to  the  entire  jumble  of  sounds. 

10.  Sometimes  when  looking  at  the  exhaust  of  a motor,  you. 
will  see  more  flame  in  one  cylinder  than  in  another,  or  different 
colored  flames.  This  may  be  an  indication  of  air  leaks  in  the 
manifold  or  weak  exhaust  valve  springs,  but  it  is  quite  likely 
to  mean  that  the  motor  is  cold  or  that  the  manifolds  do  not 
distribute  the  gas  evenly.  This  is  something  that  a mechanic 
on  the  field  cannot  correct.  It  is  well,  however,  to  watch  the 
exhaust  of  your  motor  each  day  and  if  a change  takes  place — 
if,  for  instance,  one  side  becomes  clearer  and  the  other  side 
darker,  it  may  indicate  than  an  air  leak  has  developed  or  that 
there  is  a weak  exhaust  valve  spring.  White  smoke  is  always 
due  to  an  excessive  amount  of  lubricating  oil,  but  always 
remember  that  oil  on  a plug  can  be  the  cause  of  a cylinder 
missing  by  fouling  the  spark  plug,  or  it  can  be  an  effect.  In 
other  words,  if  a spark  plug  broke  and  failed  to  deliver  a spark, 
oil  would  accumulate  in  the  cylinder  and  not  be  burned  and 
in  this  way  would  be  an  effect  and  not  a cause. 

Troubles  and  Some  of  their  Probable  Causes. 

11.  Signs  of  overheating. 

(1)  Slowing  down. 

(2)  Radiator  steaming.  Possibly  we  notice  a smell  of  burn- 
ing oil  or  hot  rubber.  Frequently  as  the  motor  begins  to  over- 
heat, vibration  will  increase,  making  the  motor  seem  to  run 


68 

harshly.  Sometimes  a smell  of  too  much  gas  or  smell  of  rich 
mixture  will  be  noticeable.  In  any  case,  stop  the  motor  as 
soon  as  you  can  when  this  is  noted,  because  continuing  to  run 
a motor  when  it  is  overheated  will  usually  ruin  it.  When  feel- 
ing the  motor  to  see  if  it  is  overheated,  remember  that  it  must 
be  felt  immediately  after  stopping  the  engine,  because  after 
the  motor  is  allowed  to  stand  a few  minutes  the  outside  be- 
comes considerably  hotter.  The  reason  for  this  is  that  the 
water  does  not  circulate  when  the  motor  is  stopped  and  be- 
comes highly  heated,  in  turn  heating  the  outer  wall  of  the  water 
jacket  and  making  the  motor  seem  very  hot.  The  radiator,  if 
felt  immediately  after  stopping  the  motor,  will  probably  feel 
about  “blood  warmth”  at  the  bottom  and  quite  hot  at  the  top, 
but  after  standing  for  some  time,  it  may  be  quite  hot  all  over. 

Some  of  the  Causes  of  Overheating. 

12.  Some  of  the  causes  of  overheating  are: 

I.  Obstructed  water  connections.  For  example,  dirt  or 
rust  flakes  in  the  pipes.  Or  frequently  in  using  hose  connec- 
tions when  the  pipe  is  thrust  into  the  hose,  the  end  of  the 
pipe  will ‘catch  and  “curl”  the  inner  layer  of  fabric  back  into 
the  hose,  in  this  way  at  least  partially  shutting  off  the  water. 

II.  Broken  piston  rings.  This  will  cause  overheating  by 
allowing  the  hot  gases  to  pass  down  between  the  piston  and 
cylinder  walls. 

III.  Leaky  radiator. 

IV.  Oil  in  the  radiator.  This  will  cause  overheating  by 
preventing  the  water  from  coming  in  actual  contact  with  the 
metal  of  the  radiator  which  is  cooled  by  the  air.  If  we  take 
two  pieces  of  iron  and  heat  them  to  a black  heat  and  put  oil 
on  one  of  them,  then  drop  both  of  them  in  a pan  of  water,  we 
may  be  able  to  pick  the  clean  one  up  immediately,  but  the  one 
which  is  oily  will  remain  hot  a great  deal  longer.  Therefore, 
never  use  an  oily  measure  to  fill  your  radiator. 

V.  A rich  mixture. 

VI.  Early  or  late  spark.  The  rich  mixture  and  the  late 
spark  cause  overheating  by  causing  the  maximum  temperature 
to  occur  late,  at  a time  when  there  is  a great  amount  of  cylin- 
der wall  exposed  to  absorb  heat,  and  therefore  this  surface 
will  be  harder  to  cool. 

VII.  Failure  of  oil  pump  or  oil  supply.  This  causes  over- 
heating on  account  of  excessive  friction  in  dry  bearings. 

VIII.  Tight  motor  or  new  parts.  If  the  bearings  are  tight 
in  a motor  or  there  are  some  tight-fitting  parts,  the  extra 
friction  may  cause  overheating. 


69 

IX.  Hot  weather.  The  cooling  systems  on  airplanes  are 
as  small  and  light  as  possible,  and  while  they  serve  to  cool 
the  motor  in  reasonable  weather,  they  may  give  trouble  from 
overheating  on  exceptionally  hot  days,  simply  because  the  size 
of  the  radiator  and  the  amount  of  water  is  not  sufficient  for 
these  extreme  conditions. 

X.  Excessive  deposits  of  carbon  or  any  other  cause  of 
pre-ignition  will  cause  overheating. 

13.  Causes  of  loss  of  power: 

I.  Pre-ignition.  Overheating  causes  loss  of  power  by  caus- 
ing pre-ignition  in  many  cases.  Remember  that  pre-ignition 
means  ignition  occurring  from  some  hot  parts  of  the  cylinder 
or  hot  particles  of  carbon,  and  occurs  far  earlier  than  the  spark 
would  occur,  causing  a tendency  for  the  motor  to  work  against 
itself. 

II.  Tight  motor. 

III.  Bad  valve  or  spark  timing. 

IV.  Broken  piston  rings. 

V.  Stoppage  in  fuel  line  or  carburetor. 

VI.  Waste  of  rags  in  the  carburetor.  Motors  in  airplanes 
draw  a large  amount  of  air  through  the  carburetor,  and  if  a 
bunch  of  rags  were  carelessly  left  under  the  hood,  it  would 
easily  be  sucked  up  by  the  carburetor  and  would  be  liable  to 
make  serious  trouble. 

VII.  Leaky  valves  or  need  of  overhauling. 

VIII.  Weak  valve  springs. 

IX.  High  altitude.  When  a motor  is  running  in  high  alti- 
tude, the  atmospheric  pressure  being  very  much  less  than  at 
sea  level,  the  piston  will  not  draw  a complete  charge.  In 
other  words,  there  is  less  difference  in  pressure  inside  the  cylin- 
der and  outside,  therefore  less  air  will  rush  in  through  the 
carburetor,  and  the  motor  never  shows  as  much  power  in 
high  altitudes  as  at  sea  level.  This  can  be  partly  corrected 
by  opening  an  auxiliary  valve  in  an  effort  to  increase  the 
amount  of  air  flowing  into  the  cylinders. 

Vibration. 

14.  Some  of  the  causes  of  vibration  are: 

I.  Pre-ignition. 

II.  Air  leaks  in  the  inlet  manifolds.  This  causes  uneven 
mixture  in  the  different  cylinders  and  therefore  a different 
action  in  the  different  cylinders  and  uneven  explosions. 

III.  Weak  valve  springs.  These  cause  vibration  by  admit- 
ting air  in  the  same  way  as  the  intake  manifold  leaks. 


70 

IV.  Broken  piston  rings.  These  cause  uneven  compression. 

V.  Rare  mixture. 

VI.  The  magneto  breaker  housing  may  be  put  on  crooked 
or  worn,  so  that  the  breaker  separates  different  distances  on 
the  two  cams,  causing  uneven  spark  timing. 

VII.  The  propeller  may  be  out  of  balance,  warped  or  “flut- 
tering.” If  the  propeller  is  built  too  lightly  it  will  flutter. 
This  cannot  be  corrected. 

VIII.  Engine-bed  bolts  loose. 

IX.  Engine  parts  of  uneven  weight.  For  example,  some 
pistons  heavier  than  others.  These  parts  must  be  as  nearly 
as  possible  the  same  weight,  usually  within  one  quarter  of  an 
ounce. 

Loss  of  Compression. 

15.  Some  of  the  causes  are: 

I.  Valve  held  open,  or  no  clearance. 

II.  Leaky  valves  or  valves  needing  grinding.  Sometimes 
the  valves  have  chunks  of  carbon  holding  them  off  their  seats. 

III.  Scored  cylinders. 

IV.  Cracked  pistons  or  cylinders. 

V.  Worn  or  broken  piston  rings. 

VI.  Cylinder  dry  or  out  of  oil.  For  example,  if  our  motor 
has  been  standing  in  the  storeroom  without  beingjyun,  all  the 
oil  may  drain  off  the  pistons  and  cylinders,  leaving  them  com- 
paratively dry,  and  poor  compression  will  result.  Some- 
times in  priming  a motor  with  gasoline  on  a cold  morning,  we 
will  get  an  excess  of  gasoline  in  the  cylinders  and  wash  the 
lubrication  out  of  it..  In  either  case,  the  remedy  is  to  put  in 
some  extra  oil  through  the  spark  plug  hole. 

VII.  Loose  spark  plug  or  bad  spark  plug  gasket. 

Failure  to  Throttle  Down,  or  Stopping  When  Throttled. 

16.  Some  of  the  causes  are: 

I.  Throttle  may  be  stuck  or  shifted  in  the  carburetor. 

II.  Throttle  controls  may  be  adjusted  wrong,  not  allowing 
the  carburetor  throttle  to  close. 

III.  Very  bad  air  leaks  in  manifold  or  weak  exhaust  valve 
springs.  In  these  cases,  the  excessive  amount  of  air  leaking 
in  will  either  stop  the  motor  when  it  is  throttled  down  or 
cause  it  to  idle  too  fast.  The  latter  trouble  would  be  in  case 
the  mixture  had  been  too  rich.  Then  the  excess  of  air  would 
thin  this  mixture  down  and  make  more  of  it.  The  result  would 
be  the  motor  would  idle  too  fast.  The  best  way  to  find  air 


71 

leaks  in  the  inlet  manifold  is  to  run  the  motor  throttled  and 
put  oil  on  each  point  with  oil  can.  It  will  be  sucked  in  where 
the  leak  is.  Don’t  use  gasoline,  because  there  is  danger  of 
fire  from  the  exhaust.  It  is  possible  to  get  enough  suction  for 
this  test  by  closing  the  throttle  and  turning  the  motor  over 
by  hand. 

IY.  Spark  plug  points  may  be  much  too  wide  apart.  In 
this  case,  it  is  difficult  for  the  magneto  to  throw  a spark  across 
the  points  at  low  speed. 

V.  Throttle  closed  too  far.  Nearly  all  carburetors  have  an 
adjustment  to  limit  the  closing  of  the  throttle,  usually  on 
the  throttle  arm. 

VI.  Too  much  or  too  little  gas,  or  in  other  words,  imperfect 
low  speed  gas  adjustment. 

VII.  Auxiliary  air  valve  may  be  loose  or  stuck  open. 

Back-Firing. 

17.  A back-fire  is  caused  either  by  the  inlet  valve  being  held 
open,  spark  occurring  while  the  valve  is  open,  or  by  a rare 
mixture. 

18.  Let  us  take  these  causes,  one  at  a time,  and  reason  them 
out. 

If  the  inlet  valve  is  held  open,  that  might  be  caused  by 
lack  of  clearance,  cam  followers  stuck,  or  valve  stuck  open. 
If  the  spark  is  occurring  while  the  inlet  valve  is  open,  it  might 
be  caused  by  the  magneto  being  timed  wrong,  the  valve  timed 
wrong,  or  the  magneto  leads  (secondary  wires)  being  crossed 
or  connected  to  wrong  cylinder.  If  the  back-fire  is  caused  by 
a rare  mixture,  remember  that  the  rare  mixture  may  be  from  either 
too  much  air  or  not  enough  gas.  If  it  is  too  much  air,  the  aux- 
iliary air  valve  may  be  stuck  open,  or  the  spring  may  be  weak. 
You  might  have  air  leaks  in  the  inlet  manifold  or  weak  exhaust 
valve  springs  admitting  air.  If  the  motor  is  not  getting 
enough  gas,  this  might  be  due  to  a clogged  air  vent  in  the  gas 
tank,  water  in  the  gasoline  pipe,  or  in  the  jets.  It  might  mean 
that  the  motor  was  cold  or  that  hot  air  was  needed. 

Loud  Exhaust. 

19.  If  the  motor  is  making  a loud  exhaust,  the  question  is, 
what  can  cause  it?  The  noise  that  we  call  the  exhaust  is 
pressure  escaping  through  the  exhaust  valve  when  it  opens. 
An  unusually  loud  exhaust  means  there  is  an  unusual  amount 
of  pressure  at  the  time  the  valve  opens.  This  might  be  caused 
by  the  valve  opening  while  there  is  high  pressure;  that  would 


72 

mean  opening  early.  The  valve  might  be  stuck  open;  a cam 
follower  turned  around;  or  if  all  the  cylinders  are  making  a loud 
exhaust,  it  might  be  the  cam  shaft  is  out  of  time. 

20.  Now  the  other  way  to  think  of  it  is  that  there  may  be 
high  pressure  at  the  time  the  valve  opens  normally.  This 
might  be  caused  by  a rich,  slow-burning  mixture,  or  from  a 
float  valve  being  held  open  by  dirt.  Possibly  the  rich  mixture 
could  be  caused  by  the  tube  through  which  the  carburetor 
sucks  the  warm  air,  being  sucked  shut.  A late  spark  can  cause 
a loud  exhaust  by  causing  the  maximum  pressure  to  occur  late, 
in  the  same  way  that  the  rich  mixture  would  cause  it. 

21.  If  all  cylinders  make  a loud  exhaust,  it  may  mean  that 
the  magneto  is  timed  wrong.  Sometimes  a loud  exhaust  in 
one  or  two  cylinders  may  be  caused  by  the  magneto  leads  or 
spark  plug  wires  being  crossed  or  mixed  up. 

Failure  to  Start. 

22.  If  the  motor  fails  to  start  the  first  thing  to  do  is  to 
inspect  the  gas  feed  carefully.  Prove  that  the  gas  flows 
through  the  passage  where  it  joins  the  carburetor,  and  prove 
that  it  flows  through  the  jets  by  running  a wire  through  them. 
If  the  trouble  is  not  found  then,  test  the  magneto  by  “ground- 
ing” a screw  driver  against  a cylinder.  Place  it  close  to  the 
top  of  the  collector  brush  or  any  place  where  we  can  get  at  the 
secondary  current  before  it  reaches  the  distributor,  to  see  if 
we  get  a spark.  If  we  fail  to  get  a spark,  remove  the  ground 
wire  and  try  it  again.  If  we  still  get  no  spark,  thoroughly  in- 
spect the  magneto  and  if  no  spark  can  be  obtained,  we  must 
have  a new  magneto,  or  at  least  a magneto  expert. 

23.  If  the  magneto  tests  out  all  right,  the  next  most  likely 
cause  of  failure  to  start  would  be  that  the  motor  is  “flooded;” 
or  in  other  words,  in  trying  to  start,  we  may  have  drawn  so 
much  gasoline  into  the  cylinders  that  it  cannot  burn.  In  this 
case,  we  must  shut  off  the  gasoline  and  close  the  switch,  open 
the  throttle  and  endeavor  to  get  a charge  of  clean  air  into  the 
cylinders;  or  in  other  words,  “air  the  motor  out.” 

24.  If  the  motor  has  been  flooded,  it  will  probably  start 
under  these  conditions.  If  we  have  found  all  these  things  all 
right,  then  the  trouble  must  be  that  either  the  spark  or  valve 
timing  is  incorrect.  In  that  case,  check  the  spark  timing; 
turn  the  engine  over  until  the  inlet  valve  on  one  cylinder  has 
just  closed;  remove  the  spark  plug  and  insert  a screw-driver  or 
wire  and  follow  the  piston  up  to  the  top  of  the  stroke;  back  it 
down  from  one-quarter  to  one-half  an  inch.  Now  by  following 


73 

the  spark  plug  wire  of  that  cylinder  to  the  distributor,  we 
should  find  the  distributor  brush  touching  the  contact  to  which 
this  wire  is  connected. 

25 . If  this  is  correct,  the  spark  timing  is  not  at  fault,  be- 
cause if  the  magneto  was  timed  close  enough  so  that  we  would 
find  the  distributor  in  contact  by  this  method,  it  would  be 
near  enough  for  the  motor  to  start,  or  at  least  for  it  to  “kick” 
one  way  or  the  other,  certainly  on  the  right  stroke.  If  we 
have  time,  we  can  measure  the  magneto  timing  more  accurately 
to  see  if  the  amount  of  advance  is  correct,  but  it  is  seldom  that 
we  find  any  great  mistake  in  the  amount  of  the  magneto  ad- 
vance, while  we  often  find  the  magneto  firing  on  the  wrong 
stroke. 

26.  If  the  valve  timing  is  suspected,  it  is  only  necessary  to 
turn  the  engine  over  and  find  out  by  watching  the  exhaust 
valve,  and  by  putting  a rod  through  the  spark  plug  hole, 
where  the  exhaust  valve  closes.  The  exhaust  valve  must  close 
on,  or  slightly  after,  the  top  center. 

Failure  to  Stop. 

27.  Sometimes  when  we  close  the  switch,  the  motor  keeps 
on  running  or  “kicking.”  If  it  merely  continues  to  kick,  the 
chances  are  that  the  motor  is  hot,  or  overheated.  Possibly 
some  particles  of  carbon,  spark  plug  points  or  exhaust  valve 
remain  red  hot  (or  incandescent),  and  are  igniting  the  charges. 
If  the  motor  continues  to  run  smoothly  after  we  close  the 
switch,  it  is  likely  that  the  ground  wire  has  become  discon- 
nected or  the  switch  fails  to  make  contact.  Possibly  the 
breaker  cap  or  cover  is  off  and  not  making  contact.  Some- 
times the  breaker  housing  is  not  put  on  far  enough.  In  this 
way  the  breaker  cover  is  held  away  from  the  breaker,  so  that 
it  cannot  short  circuit  it. 


LECTURE  IX. 

INSTALLATION,  CARE,  REMOVAL  AND  STORAGE  OF 
MOTOR. 

1 . In  placing  a rope  around  the  motor  preparatory  to  raising 
it  for  installation  in  the  fuselage,  be  careful  to  keep  the  rope 
away  from  any  part  it  might  injure.  The  water  jackets  on 
airplane  engines  are  usually  delicate  and  the  rope  must  not 
press  against  them.  Priming  cocks  are  easily  broken  off. 
Rocker  arms  are  likely  to  be  bent,  and  small  copper  or  brass 


74 

tubing  could  be  smashed  flat  by  a rope.  The  timbers  or  beds 
on  which  the  motor  will  be  placed  in  the^airplane  must  be 
level  and  parallel,  and  the  bolts  holding  them  in  the  machine 
must  be  tight. 

2.  After  the  motor  is  placed  in  them  machine,  the  switch  and 
ground  wire  should  be  connected  up  first,  so  that  the  motor 
may  be  made  “safe”  while  the  propeller  is  being  installed. 
In  making  up  the  water  connections,  be  careful  that  the  ends 
of  pipes  do  not  “turn  in”  the  inner  layer  of  fabric  in  the  hose, 
thus  obstructing  the  flow  of  water.  If  tape  is  used  on  these 
water  joints,  it  must  be  shellacked  afterwards,  because  if  the 
tape  is  not  coated  with  shellac  it  will  unwrap  and  deteriorate 
from  the  effects  of  the  hot  weather.  Make  it  a rule  never  to 
connect  the  gasoline  to  the  carburetor  without  first  allowing 
gasoline  to  flow  through  the  pipe  for  the  purpose  of  flushing  or 
cleaning  the  pipe  and  also  to  prove  the  flow  of  gasoline. 

3.  Check  the  adjustment  of  throttle  and  spark  retard  wires 
or  rods  to  see  that  they  are  properly  adjusted  to  the  motor. 
The  throttle  must  open  fully  and  close  fully,  and  when  the 
magneto  is  pulled  to  the  retard  position,  be  sure  to  see  that  the 
spring  will  pull  back  into  the  full  advance  position,  because 
if  it  fails  to  do  this,  the  motor  will  probably  overheat. 

Testing  the  Motor  on  a Testing  Block. 

4.  Airplane  motors  are  tested  after  overhauling  before  being 
installed  in  an  airplane. 

5.  Around  flying  fields  we  seldom  have  opportunity  to  test 
a motor  for  horse  power  output,  but  generally  put  a standard 
propeller  on  the  motor  or  a “club”  (dummy)  propeller  which 
will  turn  at  the  same  speed  as  the  propeller.  If  the  motor 
turns  this  standard  propeller  or  “ club  ” to  the  required  number 
of  revolutions  its  power  output  is  considered  satisfactory. 

6.  While  running  on  the  block  with  this  propeller  or  “club” 
the  motor  must: 

A.  — Run  up  to  specified  revolutions. 

B.  — Throttle  down  and  pick  up  smoothly  when  the  throttle 
is  opened. 

C.  — Not  accumulate  oil  in  the  cylinders  or  foul  spark  plugs. 

D.  — Reveal  a proper  oil-pressure  for  that  particular  motor, 
so  that  it  will  not  throw  oil.  The  motor  must  cool  properly 
and  not  have  excessive  vibration.  Must  not  slow  down  after 
running  half  an  hour  or  so,  or  seem  to  need  very  accurate 
adjustment,  and  must  not  be  “sensitive.”  Compression  must 
be  even  and  motor  must  turn  freely. 


75 


^ Care  of  the  Motor. 

7.  To  give  you  some  idea  of  how  an  airplane  motor  is  cared 
for,  I will  tell  what  is  done  to  one  make  of  motor  used  to  con- 
siderable extent  in  the  Government  Schools. 

8.  The  motor  receives  a thorough  and  careful  inspection  and 
cleaning  daily.  After  each  five  hours  running,  the  valve 
clearance  is  carefully  adjusted  to  a gauge,  spark  plugs  are 
removed,  inspected  and  adjusted.  All  the  oil  is  removed 
from  the  oil  pump  and  renewed  after  fifteen  hours  running. 
This  is  because  the  oil  comes  in  contact  with  the  hot  piston 
heads  and  deteriorates  from  the  heat;  also  becomes  filled  with 
chunks  of  carbon  and  a certain  amount  of  liquid  fuel  con- 
denses in  the  cylinder  and  mixes  with  the  cylinder  oil  im- 
pairing its  lubricating  qualities.  Remember  that  if  a motor 
is  run  too  long  without  overhauling,  when  it  is  finally  removed 
from  a machine  it  will  require  an  expensive  rebuilding  instead 
of  overhauling. 


Removal  of  Motor. 

9.  Make  it  a rule  to  remove  the  propeller  first  and  ground 
wire  connections  last.  This  is  so  that  the  motor  may  be  made 
“safe”  as  long  as  the  propeller  is  on  it.  Be  sure  that  every- 
thing is  disconnected  before  raising  the  motor  with  the  “chain” 
fall  or  tackle. 

Storage. 

10.  While  the  motor  is  in  storage,  be  sure  there  is  no  gas 
left  in  the  carburetor.  If  gas  is  left  in  the  float  chamber  of  the 
carburetor  it  will  evaporate  and  leave  a deposit  of  some  kind 
that  is  likely  to  clog  the  gas  passages  when  the  engine  is  used 
again.  Oil  should  be  put  in  the  cylinders  through  the  spark 
plug  holes,  and  the  motor  turned  over  by  hand  a few  times, 
at  least  each  week,  to  prevent  rust  in  the  cylinders;  and  the 
valve  stems  should  be  oiled  with  an  oil  can. 

Overhauling. 

11.  Overhauling  is  a very  elastic  word.  It  can  mean  merely 
disassembly  and  reassembly  of  a motor,  or  it  can  mean  very 
carefully  taking  a motor  apart  and  noticing  each  place  where  it 
is  wearing  or  rubbing  and  investigating  the  cause,  and  careful 
planning  to  correct  and  improve  conditions  while  assembling 
the  motor. 


76 

12.  One  important  thing  is  to  trace  the  lubrication  system 
from  the  oil  pump  or  through  the  motor  and  back  to  the  sump. 
After  learning  the  route  through  which  it  travels,  all  passages 
should  be  carefully  cleaned  out  by  forcing  gasoline  through 
them  to  avoid  any  possibility  of  stoppage  in  the  oil  circulation. 

13.  After  a motor  is  overhauled,  it  is  best  to  make  two  or 
three  short  flights,  for  instance  10  or  15  minute  flights,  before 
a long  flight  is  made,  and  the  motor  should  be  examined  or 
felt  after  each  of  these  short  flights.  It  is  far  better,  if  pos- 
sible, to  install  the  motor  on  a block  and  test  it  thoroughly 
before  installing  it  in  an  airplane. 


LECTURE  X. 

DIFFERENCES  BETWEEN  AIRPLANE  AND  AUTOMO- 
BILE ENGINES. 

1.  First  of  all,  airplane  engines  are  always  built  lightly, 
and  this  means  that  the  crank  case  is  not  very  rigid.  The 
timbers  or  beds  on  which  the  engine  sits  must  be  carefully 
lined  up  and  be  perfectly  true  so  that  the  crank  case  will  not 
be  sprung  out  of  shape  when  it  is  bolted  to  them. 

2.  Airplane  engines  run  at  nearly  full  capacity  all  the  time. 
This  means  that  we  have  full  compression,  maximum  tempera- 
tures and  maximum  pressures,  also  maximum  vibration  all  the 
while.  Lubrication  of  moving  parts  becomes  a greater  prob- 
lem under  these  conditions.  For  example,  in  a racing  auto- 
mobile, the  driver  throttles  down  his  motor  to  a certain  extent 
on  the  turns.  This  will  allow  the  crank  shaft  to  ease  up  on  its 
bearings  slightly,  allowing  the  oil  film  to  be  renewed  under  the 
shaft.  In  the  airplane  motor  there  is  no  such  easing  up  on 
turns.  Airplane  engines  run  at  high  temperatures  because  of 
the  necessarily  light  weight  of  the  cooling  systems.  In  other 
words,  the  cooling  systems  are  very  close  to  the  limit.  Air- 
plane motors  are  subject  to  strains  caused  by  the  propeller. 
Even  the  best  built  airplane  propellers  vibrate  or  flutter  to  a 
certain  extent  and  all  this  vibration  goes  through  the  motor. 
Usually  the  crank  shaft  gets  it  first  and  when  flying,  the  gusty 
winds  impose  great  strains  on  the  propeller  and  crank  shaft 
due  to  the  uneven  air  through  which  the  propeller  is  working. 
As  the  airplane  is  “pushed”  or  “lead”  through  the  air  by  its 
crank  shaft  in  most  cases,  it  is  necessary  to  have  a substantial 
thrust  bearing  on  the  crank  shaft  to  take  care  of  these  strains. 


77 

3.  It  is  somewhat  difficult  to  tune  up  an  airplane  motor  on 
the  ground  for  the  following  reasons;  The  motor  cannot  he 
run  long  on  the  ground  without  overheating  because  the  air- 
plane is  stationary  and  the  propeller  throws  little  or  no  air 
through  the  radiator.  (The  central  part  of  the  propeller  is 
not  designed  to  do  any  work  or  throw  any  air  to  amount  to 
anything  and  the  result  is  that  it  throws  practically  no  air 
through  the  radiator  when  the  machine  is  standing  still.) 
The  result  is,  if  we  are  tuning  up  the  motor  we  can  run  it  with 
throttle  open  for  short  intervals. 

4.  In  some  machines  there  is  a difference  in  the  atmospheric 
conditions  around  the  carburetor  when  the  machine  is  stand- 
ing or  flying.  For  instance,  when  the  machine  is  flying,  there 
may  be  either  a decrease  or  an  increase  in  the  atmospheric 
pressure,  due  to  the  location  of  the  carburetor,  and  this  makes 
carburetor  adjustment  on  the  ground  difficult  in  some  instances. 

5.  Airplane  engines  must  run  at  high  altitudes  and  under 
these  conditions  encounter  very  low  temperatures  and  very 
low  atmospheric  pressure,  which  means  that  the  cylinders 
will  not  get  a complete  charge  of  gas.  A motor  loses  a large 
percentage  of  its  power,  even  at  a 10,000  foot  altitude.  The 
reason  is  that  there  is  less  air  pressure  outside  the  motor; 
therefore  less  air  or  mixture  rushing  through  the  carburetor, 
and  into  the  cylinder  during  the  suction  stroke.  One  of  the 
remedies  for  this  is  to  open  an  extra  air  passage  in  the  inlet 
manifold  to  allow  a larger  volume  of  air  to  enter  the  cylinders. 
Other  methods  are  being  experimented  with. 

6.  Airplane  engines  must  run  at  all  angles.  This  means 
there  must  be  plenty  of  “fall”  from  the  gasoline  tank  to  the 
carburetor.  The  oil  sumps  of  the  crank  case  must  have  the 
oil  continually  pumped  out  of  them  because  a large  accumula- 
tion of  oil  in  the  crank  case  would  flood  the  cylinders  under 
certain  flying  conditions.  Some  of  the  exhibition  flyers  have 
had  carburetors  especially  arranged  to  feed  gasoline  while  the 
airplane  is  upside  down.  Such  devices  are  not  in  common 
use,  however, 

7.  You  probably  know  that  if  we  take  “hay-wire”  or  soft 
iron  wire  and  bend  it  several  times  it  becomes  brittle  and 
breaks.  This  is  known  as  crystallization.  In  airplanes,  if 
the  engine  vibrates,  it  sends  little  strains  through  the  entire 
airplane  which  bend  the  metal  parts  ever  so  slightly.  As 
these  vibrations  occur  frequently  and  for  long  periods,  we  find 
the  metal  in  time  becomes  crystallized  in  the  same  way  as  the 
soft  iron  wire  I spoke  of.  The  result  may  be  that  while  the 
machine  is  “taxying”  along  on  the  ground  to  the  hangar, 


78 

fittings  may  snap  simply  because  thay  are  crystallized.  I 
mention  this  so  that  you  will  know  the  importance  of  keeping 
a motor  running  smoothly.  On  an  airplane  motor  the  com- 
pression in  all  cylinders  must  be  uniform.  Moving  parts  must 
all  weigh  alike.  Spring  tension  must  be  uniform.  Spark 
plug  points  must  be  adjusted  at  uniform  gaps.  If  more  than 
one  magneto  is  used,  they  must  be  carefully  synchronized  or 
timed  together  and  certainly  all  the  plugs  must  be  of  the  same 
make  and  type. 

8.  Airplane  engines  must  be  free  from  vibration,  because 
engine  vibration  rapidly  crystallizes  all  the  metal  parts  in  the 
airplane,  which  means  that  they  are  liable  to  break  without 
being  subjected  to  any  great  strains. 

9.  Most  American  airplane  engines  are  built  with  the  car- 
buretor very  low.  While  this  gives  a long  inlet  manifold 
which  has  some  disadvantages,  it  has  the  advantage  of  placing 
the  carburetor  where  it  is  easily  fed  from  a tank  located  in  the 
fuselage  of  the  airplane.  Many  European  machines  have  a 
gas  tank  placed  in  or  under  the  top  plane,  but  this  necessi- 
tates pipes  running  up  to  this  tank  and  consequently  increased 
danger  from  leaks  and  possible  fire. 

10.  A good  many  air  pressure  gas  feed  systems  are  used  with 
good  success;  and  also  gasoline  pumps,  pumping  an  excessive 
amount  of  gasoline  to  a tank  which  will  feed  the  carburetor  by 
gravity.  This  tank  has  an  over-flow  pipe  to  the  main  storage 
tank,  to  take  care  of  the  excess  delivered  by  the  pump.  Re- 
member that  gasoline  tanks  in  airplanes  must  have  “ baffle 
plates”  to  keep  the  gasoline  from  washing  from  one  end  to  the 
other.  One  reason  for  this  is  that  if  gasoline  could  rush  from 
one  end  to  the  other,  it  might  seriously  affect  the  balance  of  the 
airplane.  Air  vents  in  gasoline  tanks  must  be  so  arranged  that 
should  gasoline  splash  out  of  them  it  could  not  be  ignited  by  the 
hot  gases  from  the  exhaust  pipes.  Frequently  these  vents 
have  a tube  running  down  through  the  bottom  of  the  fuselage 
for  safety. 

11.  Up  to  the  present  time  most  airplane  motors  have  been 
equipped  with  exhaust  “ stacks”  (short  pipes).  The  purpose 
of  these  stacks  is  to  carry  the  gases  away  from  the  fuselage, 
and  also  to  have  a slight  syphon  effect  to  scavenge  the  cylin- 
ders, but  at  present  many  airplanes  are  equipped  with  pipes 
to  carry  the  exhaust  gases  and  sounds  above  the  upper  plane. 

12.  It  is  difficult  to  use  mufflers  on  airplane  engines,  partly 
due  to  the  fact  that  a muffler  reduces  the  power  of  a motor, 
but  more  particularly  because  an  engine  is  more  difficult  to 


79 

cool  if  it  is  equipped  with  a muffler,  and  the  exhaust  is  more 
liable  to  burn.  « 

13.  The  gaskets  between  the  exhaust  pipes  and  cylinders 
are  important.  If  they  blow  out,  there  is  danger  of  the  flame 
coming  through  and  burning  some  part  of  the  airplane,  or  in 
some  motors,  this  flame  could  ruin  the  exhaust  valve  springs 
by  overheating  them  and  causing  them  to  collapse. 

14.  Gasoline  strainers  and  settlers  must  be  arranged  so  that 
they  will  work  properly  with  the  airplane  in  various  angles. 
Many  settlers  now  in  use  fail  in  this  respect. 

BACK-FIRING. 

15.  Back-firing  in  the  carburetor  is  particularly  dangerous 
in  airplanes.  Some  manufacturers  are  experimenting  with 
devices  to  prevent  back-firing.  If  an  airplane  catches  fire,  it 
is  a very  dangerous  situation  for  the  pilot  because  he  cannot 
step  out  on  the  ground  and  use  the  fire  extinguisher.  If  he  is 
up  in  the  air  he  is  likely  to  be  burned  badly  before  he  can  land 
the  machine,  therefore,  every  precaution  should  be  taken  to 
avoid  chance  of  fire. 


